Make KDC "status" statements more homogeneous

Generally we want KDC status strings to be concise, informative and follow
some common rules:

- All letters in the status string should be capitalized;
- the words in the status phrase are separated by underscore;
- abbreviations should be avoided.  Some acceptable "standard" acronyms
  are AS_REQ, TGS_REP etc.
- since in almost all cases KDC status is set on error, no need
  to state this fact as part of the status string;
- KDC status string should be an imperative phrase.

For example, "DECRYPT_SERVER_KEY".

This commit is to modify some KDC status messages to follow this format.

Even though KDC status messages are not standardized, it is possible that some
administrators use them in the Kerberos log file processing. Hence, the vast
majority of them are left unchanged pending further investigation (mostly,
feedback from the administrators).

Reverted some code to the original state

do_as_req: "FETCH_LAST_REQ"  and code around NULL_SERVER
do_tgs_req: "UNEXPECTED NULL in header_ticket" is left alone for now