Fix out-of-tree builds

Had to generate (with sed) enterprise_extension.[ch] in source (not
object) directory, because of circular dependency madness. Add all
auto-generated files to CLEANFILES for proper make-clean.
Fix typo in "allowciphers" server control option parsing.
Don't register lock cleanup if WriteLock() failed.

This way we avoid spurious error messages on exit, from when trying to
clear the inexistent lock (minor annoyance seen in the past while
stress-killing cf-serverd).
Turn some "should not happen" comments to UnexpectedError().
Merge pull request #968 from jimis/master

Various small fixes/cleanups