top: introduce background updates via separate threads

After the stage had been set in the previous patch, in
this patch we will actually implement those background
updates via 3 separate threads. The design was simple:

. the do-while loops have now been made truly infinite
. 2 semaphores per thread allow needed synchronization
. 1 semaphore will provide for each thread to sem_wait
. 1 semaphore will provide for display o/p to sem_wait
. and all 3 thread's program name was made descriptive

A complication was the potential for a signal directed
to one of our new threads. Rather than having a thread
try to deal with such signals, we pass a mask with all
signals blocked at pthread_create time. Thereafter any
subsequent signals are forwarded to the parent thread.

[ also sigprocmask was exchanged for pthread_sigmask ]
[ since warned about use "in multithreaded process". ]

[ plus we also modified each of those POSIX comments ]
[ about 2004 to agree with current signal-safety(7). ]

Sadly, after all this effort there were no performance
benefits to having separate threads. In fact there was
a measurable performance degradation when running with
ever smaller delay intervals. But even with a delay of
1/10 second the 'real' cost increase is only about 1%.

There is one way whereby any additional costs might be
eliminated (at least seemingly). One could introduce 2
separate sets of contexts for each of those 3 threads.
Then retrieval & display could be overlapped. However,
the resulting display wouldn't represent the real-time
results. Rather it would be stale by 1 delay interval.

Signed-off-by: Jim Warner <james.warner@comcast.net>

top/top_nls.h

@@ -84,6 +84,7 @@ enum norm_nls {
    WORD_process_txt, WORD_threads_txt, WRITE_rcfile_fmt, WRONG_switch_fmt,
    XTRA_badflds_fmt, XTRA_fixwide_fmt, XTRA_modebad_txt, XTRA_size2up_txt,
    XTRA_vforest_fmt, XTRA_warncfg_txt, XTRA_warnold_txt, XTRA_winsize_txt,
+   X_SEMAPHORES_fmt, X_THREADINGS_fmt,
 #ifndef INSP_OFFDEMO
    YINSP_demo01_txt, YINSP_demo02_txt, YINSP_demo03_txt, YINSP_deqfmt_txt,
    YINSP_deqtyp_txt, YINSP_dstory_txt,