signing: move to new signing verification and return scheme

signing: move to new signing verification and return scheme

This gives us more granularity than the former Never/Optional/Always
trifecta. The frontend still uses these values temporarily but that will
be changed in a future patch.

* Use 'siglevel' consistenly in method names, 'level' as variable name
* The level becomes an enum bitmask value for flexibility
* Signature check methods now return a array of status codes rather than
  a simple integer success/failure value. This allows callers to
  determine whether things such as an unknown signature are valid.
* Specific signature error codes mostly disappear in favor of the above
  returned status code; pm_errno is now set only to PKG_INVALID_SIG or
  DB_INVALID_SIG as appropriate.

Signed-off-by: Dan McGee <dan@archlinux.org>