Avoid GLib.

Prepare to migrate away from GLib.  Currently, we're still using GLib
as a backend.

Replaces GLib's GError.

g_file_error_quark() is meant to be used with the GFileError enum
which does not correspond with errno, but must be converted with
g_file_error_from_errno().

At the same time, this removes g_strerror() use for
g_file_error_quark().

Prepare for splitting command.c.