Various cleanups: threads, STA_NANO

[Hal Murray]

We have various cruft associated with threads.  Can we add POSIX threads to 
our list of requirements?  Or is it already included in POSIX.1-2001 and 
ISO/IEC 9899:1999 (C99)?

The idea is to remove HAVE_PTHREAD_H and HAVE_PTHREAD from config.h and remove 
most of wafhelpers/check_pthread.py

wafhelpers/check_pthread.py says:
    ctx.check_cc(lib="thr", mandatory=False,
                 comment="thr library, required by some operating systems.")
Is that (still?) correct?  If so, which systems?

There is probably cruft in this area that I added to try to avoid libpthread 
for the no-DNS case.  That doesn't work now since libcrypto needs pthread_once 
and is less likely to work as we get NTS-KE working.

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We have various ifdefs for STA_NANO.  We depend on clock_gettime which uses 
timespec so all the systems we run on know about nanoseconds.  Can we upgrade 
our requirement for ntp_adjtime to be requires ntp_adjtime with STA_NANO?


-- 
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