fork() and system(): Change errno value from EAGAIN to ENOSYS

This is more correct.  Trying again will not help.

Fixes: emscripten-core#14124

Author: ijackson

src/library.js

@@ -171,7 +171,7 @@ LibraryManager.library = {
     // pid_t fork(void);
     // http://pubs.opengroup.org/onlinepubs/000095399/functions/fork.html
     // We don't support multiple processes.
-    setErrNo({{{ cDefine('EAGAIN') }}});
+    setErrNo({{{ cDefine('ENOSYS') }}});
     return -1;
   },
   vfork: 'fork',
@@ -411,7 +411,7 @@ LibraryManager.library = {
     // http://pubs.opengroup.org/onlinepubs/000095399/functions/system.html
     // Can't call external programs.
     if (!command) return 0; // no shell available
-    setErrNo({{{ cDefine('EAGAIN') }}});
+    setErrNo({{{ cDefine('ENOSYS') }}});
     return -1;
   },