Convert k5-thread macros to functions

Convert k5-thread macros to functions

k5-thread.h has several pthread support calls defined as macros which
conditionally call pthread functions.  If a program is linked with
libkrb5support and uses these macros, and the program isn't compiled
with -fPIC, then it can crash if the pthread functions are linked at
runtime (via LD_PRELOAD, for instance) but not at compile time.
Convert the conditional macros to functions, so that libkrb5support is
responsible for determining whether pthreads is loaded and for calling
the pthreads functions if it is.

[ghudson@mit.edu: clarified commit message, adjusted whitespace]