chg: rewrite ssiClose_thread()

+ add movePointerOn()
+ add timespec_subtract()
+ rewrite ssiCloseList_append()
+ use ssiCloseList_last instead of ssiCloseList_first to signal shutdown
+ remove ssiCloseList_treat
rmv: remove ssiCloseSingleLink()
fix: do not use pthread functions in signal handler

These functions are not async-safe.
+ remove shutdownMutex
chg: raise RLIMIT_NPROC if defined and necessary
chg: correct some comments
chg: use pointers for mutexes
chg: change handling of mutexes in ssiSignal_CloseSuccessful()
chg: introduce ssiGet_WaitForClose_thread()
chg: rewrite pthread_atfork_child()
fix: in ssiKill, remove link from ssiCloseList
fix: defer shutdown in slKill() and slCleanUp()
fix: handle already killed links in ssiCloseList correctly
fix: do not call m2_end() twice
fix: set ssiToBeClosed to NULL in m2_end()
fix: use close() instead of fclose() for parent of fork
chg: make 2nd and 3rd arg of timespec_subtract() const