add sendmail's sysexits.h return codes to the LSB

[Jeff Johnson]

The error return codes used by the original sendmail are widely copied, and
provide more complete information about the failure of mail delivery than the
POSIX sendmail spec's "nonzero == error" requirement.

There's been a request in lsb-discuss for better error handling for sendmail,
and these codes have the advantage of already being best practice. Most
likely, we wouldn't even have to request changes of the major MTAs.

Here's one online spec for these codes, though I'm not sure if it's appropriate
to use as *the* spec:

https://developer.apple.com/library/mac/documentation/Darwin/Reference/Manpages/man3/sysexits.3.html

Mailing list thread:

http://lists.linuxfoundation.org/pipermail/lsb-discuss/2013-July/007677.html