config.c: Log #exec include failures.

config.c: Log #exec include failures.

If the script referenced by `#exec` does not exist, writes anything to
stderr, or exits abnormally or with a non-zero exit status, we log
that to Asterisk's error logging channel.

Additionally, write out a warning if the script produces no output.

Fixes #259

(cherry picked from commit b437cc3267705b01278da404bf9322cadc5be995)