libc: add '__get_fdpath' function

This function allows for querying the underlying 'fd_path' of a given
libc fd.

So far it is only used by Haiku-on-Genode and the initial implementation
was provided by Cedric.

Issue genodelabs/genode-world#354.

Author: cnuke

repos/libports/lib/symbols/libc

@@ -15,6 +15,7 @@ __error T
 __flt_rounds T
 __fpclassifyd T
 __fpclassifyf T
+__get_fdpath T
 __h_errno T
 __h_errno_set T
 __has_sse D 4

repos/libports/src/lib/libc/file_operations.cc

@@ -805,3 +805,17 @@ extern "C" int __getcwd(char *dst, ::size_t dst_size)
 	copy_cstring(dst, cwd().base(), dst_size);
 	return 0;
 }
+
+
+extern "C" int __get_fdpath(int libc_fd, char *dst, ::size_t dst_size)
+{
+	using namespace Libc;
+
+	File_descriptor *fd = file_descriptor_allocator()->find_by_libc_fd(libc_fd);
+	if (fd == 0)
+		return Errno(ENOTSUP);
+
+	copy_cstring(dst, fd->fd_path, dst_size);
+
+	return 0;
+}