mailwrapper(8): change behaviour if mailer.conf cannot be opened

Previously, mailwrapper(8) would default to invoking _PATH_DEFAULTMTA
(i.e., dma) if mailer.conf couldn't be opened for any reason, including
transient errors like ENFILE. This behaviour is undesirable, because if
the administrator has configured a different MTA in mailer.conf, they
almost certainly don't want mailwrapper to unpredictably fall back to
the compiled-in default; and in any case, the default MTA is probably
not running, meaning the mail may be queued and then never delivered,
which is worse than not accepting it to begin with.

Change this behaviour depending on why mailer.conf can't be opened:

• If it doesn't exist, keep the existing behaviour of falling back to the default MTA, on the assumption that this is a reasonable default if mailer.conf hasn't been configured at all.

• If it cannot be opened for any other reason, do not invoke an MTA and instead return an error to the caller.

PR: 25218
Reviewed by: imp, emaste, markj
Pull Request: https://github.com/freebsd/freebsd-src/pull/969