sslsocket: fix "attempt to redefine 'SSL_*'" error

Different libraries can use FFI OpenSSL bindings.
E.g. cartridge already uses it:
https://github.com/tarantool/cartridge/blob/master/cartridge/sslsocket.lua

Right now there is no any significant difference in cdef. But in
future it can be different (e.g. after implementation of #207).
Since we use "struct SSL_METHOD {} SSL_METHOD" definition
it can cause errors like 'attempt to
redefine 'SSL_METHOD'" that lead to the case then pcall silently
handle an error but some symbols won't be defined.

This patch fixes such case.
That's only relevant for struct definitions. Functions and typedefs can be redefined.

Author: olegrok

http/sslsocket.lua

@@ -9,55 +9,52 @@ local buffer = require('buffer')
 local clock = require('clock')
 local errno = require('errno')
 
-pcall(
-    function()
-        ffi.cdef[[
-            typedef struct SSL_METHOD {} SSL_METHOD;
-            typedef struct SSL_CTX {} SSL_CTX;
-            typedef struct SSL {} SSL;
+pcall(ffi.cdef, 'typedef struct SSL_METHOD {} SSL_METHOD;')
+pcall(ffi.cdef, 'typedef struct SSL_CTX {} SSL_CTX;')
+pcall(ffi.cdef, 'typedef struct SSL {} SSL;')
 
-            const SSL_METHOD *TLS_server_method(void);
-            const SSL_METHOD *TLS_client_method(void);
+pcall(ffi.cdef, [[
+    const SSL_METHOD *TLS_server_method(void);
+    const SSL_METHOD *TLS_client_method(void);
 
-            SSL_CTX *SSL_CTX_new(const SSL_METHOD *method);
-            void SSL_CTX_free(SSL_CTX *);
+    SSL_CTX *SSL_CTX_new(const SSL_METHOD *method);
+    void SSL_CTX_free(SSL_CTX *);
 
-            int SSL_CTX_use_certificate_file(SSL_CTX *ctx, const char *file, int type);
-            int SSL_CTX_use_PrivateKey_file(SSL_CTX *ctx, const char *file, int type);
-            void SSL_CTX_set_default_passwd_cb_userdata(SSL_CTX *ctx, void *u);
-            typedef int (*pem_passwd_cb)(char *buf, int size, int rwflag, void *userdata);
+    int SSL_CTX_use_certificate_file(SSL_CTX *ctx, const char *file, int type);
+    int SSL_CTX_use_PrivateKey_file(SSL_CTX *ctx, const char *file, int type);
+    void SSL_CTX_set_default_passwd_cb_userdata(SSL_CTX *ctx, void *u);
+    typedef int (*pem_passwd_cb)(char *buf, int size, int rwflag, void *userdata);
 
-            void SSL_CTX_set_default_passwd_cb(SSL_CTX *ctx, pem_passwd_cb cb);
+    void SSL_CTX_set_default_passwd_cb(SSL_CTX *ctx, pem_passwd_cb cb);
 
-            int SSL_CTX_load_verify_locations(SSL_CTX *ctx, const char *CAfile,
-                                            const char *CApath);
-            int SSL_CTX_set_cipher_list(SSL_CTX *ctx, const char *str);
-            void SSL_CTX_set_verify(SSL_CTX *ctx, int mode,
-                                    int (*verify_callback)(int, void *));
+    int SSL_CTX_load_verify_locations(SSL_CTX *ctx, const char *CAfile,
+                                      const char *CApath);
+    int SSL_CTX_set_cipher_list(SSL_CTX *ctx, const char *str);
+    void SSL_CTX_set_verify(SSL_CTX *ctx, int mode,
+                            int (*verify_callback)(int, void *));
 
-            SSL *SSL_new(SSL_CTX *ctx);
-            void SSL_free(SSL *ssl);
+    SSL *SSL_new(SSL_CTX *ctx);
+    void SSL_free(SSL *ssl);
 
-            int SSL_set_fd(SSL *s, int fd);
+    int SSL_set_fd(SSL *s, int fd);
 
-            void SSL_set_connect_state(SSL *s);
-            void SSL_set_accept_state(SSL *s);
+    void SSL_set_connect_state(SSL *s);
+    void SSL_set_accept_state(SSL *s);
 
-            int SSL_write(SSL *ssl, const void *buf, int num);
-            int SSL_read(SSL *ssl, void *buf, int num);
+    int SSL_write(SSL *ssl, const void *buf, int num);
+    int SSL_read(SSL *ssl, void *buf, int num);
 
-            int SSL_pending(const SSL *ssl);
+    int SSL_pending(const SSL *ssl);
 
-            void ERR_clear_error(void);
-            char *ERR_error_string(unsigned long e, char *buf);
-            unsigned long ERR_peek_last_error(void);
+    void ERR_clear_error(void);
+    char *ERR_error_string(unsigned long e, char *buf);
+    unsigned long ERR_peek_last_error(void);
 
-            int SSL_get_error(const SSL *s, int ret_code);
+    int SSL_get_error(const SSL *s, int ret_code);
 
-            void *memmem(const void *haystack, size_t haystacklen,
-                    const void *needle, size_t needlelen);
-        ]]
-    end)
+    void *memmem(const void *haystack, size_t haystacklen,
+                 const void *needle, size_t needlelen);
+]])
 
 local function slice_wait(timeout, starttime)
     if timeout == nil then