Removed all the "enum foo;" declarations. They didn't seem to be very standard. Old gcc versions warn about them and C++ code doesn't like them.

doveadm: Improved error handling. Failures should now always have non-zero exit code. doveadm now uses sysexits.h exit codes in most places, although there are still a lot of places where it simply returns EX_TEMPFAIL even though something else might be better.