[krbdev.mit.edu #3417] move/add thread support to support lib

[Ken Raeburn via RT]

For use by plugin code, the thread support interface layer (with the
krb5int_ names, not the k5_ shorthand names) has to be exported from the
support library.  New functions to allocate/free mutexes will also be
needed.

It might be easiest, though not exactly ideal for performance, to have
all our code call the support-library versions, and drop most of the
complexity from the header file.  We could perhaps drop all of it if we
changed everything to use mutex pointers instead of mutexes inside other
data structures, but that's more work.  For now, some wrapper-function
hacks to export "what the macros do" from the library should suffice.