Add new function krb5_boolean krb5_is_thread_safe(void) to library.

  Add new function krb5_boolean krb5_is_thread_safe(void) to library.
  This function simply reports whether or not ENABLE_THREADS was specified
  at compile time.  It is meant to be used by applications which need to
  know whether or not multithread libraries must be initialized OR
  whether or not calls to Kerberos library functions from multi-threaded
  applications must be protected by an application global mutex.

ticket: new
tags: pullup

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@17003 dc483132-0cff-0310-8789-dd5450dbe970