Add missing function names in the NAME section for rest of the man pages in libpthread

Fix some lint.

Too much or too little whitespace;
improve macro usage;
add missing .El;
merge error sections for same error code.

Mark up (unsupported) PTHREAD_PROCESS_SHARED with .Dv macro.

Use

.Vt type var No = Dv INITIALIZER;

to provide examples of static initialization in SYNOPSIS section.
.Va macro does the wrong thing (check groff PostScript output) and the
need for .Pp kludge

Use

.Vt type var No = Dv INITIALIZER;

to provide examples of static initialization in SYNOPSIS section.
.Va macro does the wrong thing (check groff PostScript output) and the
need for .Pp kludge (now dropped) should have been an indication too.

While here move static initialization example to be right after the
declaration of *_init().

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Mark up NULL.

Add comma in enumeration, sort errors.

Merge/group man pages:
- pthread_barrier_{init,destroy,wait} into pthread_barrier(3).
- pthread_cond_* into pthread_cond(3).
- pthread_mutex_* into pthread_mutex(3).
- pthread_rwlock_* into pthread_r

Merge/group man pages:
- pthread_barrier_{init,destroy,wait} into pthread_barrier(3).
- pthread_cond_* into pthread_cond(3).
- pthread_mutex_* into pthread_mutex(3).
- pthread_rwlock_* into pthread_rwlock(3).
- pthread_spin_* into pthread_spin(3).
Cross-link amongst groups.
OK jruoho@

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