Use error_message(errno) instead of using sys_errlist[errno]. This

Use error_message(errno) instead of using sys_errlist[errno].  This
avoids the hair of deciding whether or not we need to declare
sys_errlist or use strerror(), etc., since com_err has all of that
complexity anyway.  Also fixed lots of -Wall nits.

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@10268 dc483132-0cff-0310-8789-dd5450dbe970