[krbdev.mit.edu #1303] thread safety in mit-krb5 libraries

[Ken Raeburn via RT]

Many changes are needed in the krb5 library (and others) to make the
code thread-safe.

Any non-const global variables need protection against simultaneous read
and write.  (If a static-duration object can't be updated, it should
probably be const.)

File locking is per-process (at least under UNIX); when we do file
locking, we'll want it to effectively be per-thread.

This issue has been discussed in email (on krbdev?), and people at IBM
are supposed to be writing some code based on that discussion; just
making sure it gets listed here too....