Make sure non-aborting signal handlers save and restore errno

Make sure non-aborting signal handlers save and restore errno

If an async signal interrupts some function, we can have this
anti-pattern:

    /* in normal code */
    result = some_syscall (); /* fails, e.g. errno = EINVAL */

        /* interrupted by async signal handler */
        write (...); /* fails, e.g. errno = ENOBUFS */

    /* back to normal code */
    if (errno == EINVAL) /* problem! it should be but it isn't */

The solution is for signal handlers to save and restore errno.

This is unnecessary for signal handlers that can't touch errno (like
the one in dbus-launch that just sets a flag), and for signal handlers
that never return (like the one in test-utils-glib for timeouts).

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=103010
Signed-off-by: Simon McVittie <smcv@collabora.com>
Reviewed-by: Philip Withnall <withnall@endlessm.com>