Add config option to use STGRTMIN-based signals for thread suspend/resume

Change motivated by using BDWGC in the X10 runtime on systems where
IBM Parallel Environment is used to launch the X10 processes. PE
is already using the signals BDWCG is using for its own purposes, so
we need to force the GC to use different ones to avoid a conflict.

* configure.ac (sigrt-signals): Handle new option.
* configure.ac (GC_USESIGRT_SIGNALS): New AC define.
* include/private/gc_priv.h (SIG_SUSPEND): Force to be defined to
_SIGRTMIN+6 if GC_USESIGRT_SIGNALS (Darwin is excluded anyway).
* pthread_stop_world.c (SIG_THR_RESTART): Force to be defined to
_SIGRTMIN+5 if GC_USESIGRT_SIGNALS; reformat code.
Improve documentation for disappearing links in gc.h

* include/gc.h (GC_free): Add comment about disappearing links.
* include/gc.h (GC_general_register_disappearing_link): Add comment
about the conditions for a link to disappear.
Fix 'e.g.' typos in comments and documentation

* doc/README.cmake: Fix typo (e.g.).
* doc/README.solaris2: Likewise.
* doc/README.solaris2: Likewise.
* doc/README.win32: Likewise.
* include/gc.h (GC_register_my_thread): Fix typo in a comment (e.g.).
* include/leak_detector.h: Likewise.
* misc.c (GC_VSNPRINTF): Likewise.
* pthread_stop_world.c (GC_unblock_gc_signals): Likewise.
Fix GC_sig_suspend initialization when non-constant SIGRTMIN used

* pthread_stop_world.c (SIGNAL_UNSET): New macro.
* pthread_stop_world.c (GC_sig_suspend, GC_sig_thr_restart): Initialize
to SIGNAL_UNSET (instead of SIG_SUSPEND/THR_RESTART).
* pthread_stop_world.c (GC_get_suspend_signal): Return SIG_SUSPEND if
GC_sig_suspend is SIGNAL_UNSET.
* pthread_stop_world.c (GC_get_thr_restart_signal): Return
SIG_THR_RESTART if GC_sig_thr_restart is SIGNAL_UNSET.
* pthread_stop_world.c (GC_unblock_gc_signals): Add assertion for
initialization of GC_sig_suspend and GC_sig_thr_restart.
* pthread_stop_world.c (GC_stop_init): Replace SIGNAL_UNSET value for
GC_sig_suspend, GC_sig_thr_restart to SIG_SUSPEND and SIG_THR_RESTART,
respectively.