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().

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/cc @walters @alexl @pwithnall