Only system headers should declare errno

Only system headers should declare errno

Many parts of krb5 declare errno in source files using extern int
errno.  This has not been needed in ANSI C for a long time,
and actively breaks modern versions of glibc.

So these declarations are removed and errno.h included where necessary.

Ticket: new
Component: krb5-build

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@15082 dc483132-0cff-0310-8789-dd5450dbe970