Fix thread support for Solaris and simplify

Fix thread support for Solaris and simplify

threads.c failed to build on Solaris afer commit
17932091cc0d5981c5a78d389ffa4a5c7b532bd6 because k5-thread.h did not
define the conditional k5_once_t structure (because NO_WEAK_PTHREADS
is defined) but threads.c tried to build the conditional k5_once()
function.

Use a single preprocessor symbol, USE_CONDITIONAL_PTHREADS, to
determine whether to use and define pthreads functions which
conditionalize on whether pthreads is loaded.  In threads.c, move the
new k5_once() definitions into the USE_CONDITIONAL_PTHREADS block,
defining a stub function if other code will not refer to it.

Also move #pragma weak declarations from k5-threads.h into threads.c,
as we should no longer be conditionally referring to those symbols
outside of threads.c.

Also eliminate some missing-prototype warnings where we define
functions for linker-visibility but don't have corresponding
prototypes in k5-threads.h.