gstdio: Add g_clear_fd() and g_autofd

Inspired by libglnx's glnx_close_fd() and glnx_autofd, these let us
have the same patterns as g_clear_object() and g_autoptr(GObject), but
for file descriptors. g_clear_fd() is cross-platform, while g_autofd
is syntactic sugar requiring a supported compiler (gcc or clang).

Now that g_close() checks for EBADF as a programming error, we can
implement the equivalent of glnx_autofd as an inline function without
needing to have errno and EBADF in the header file.

g_clear_fd() is like glnx_close_fd(), but with error checking.
The private _g_clear_fd_ignore_error() function used to implement
g_autofd is a closer equivalent of glnx_close_fd().

Signed-off-by: Simon McVittie <smcv@collabora.com>

docs/reference/glib/glib-sections.txt.in

@@ -1454,6 +1454,8 @@ g_creat
 g_chdir
 g_utime
 g_close
+g_clear_fd
+g_autofd
 
 <SUBSECTION Private>
 g_file_error_quark