chg: remove signal handler for SIGCHLD
chg: globally block SIGCHLD
add: mutex and condition variable for ssiClose
chg: split ssiClose() up into ssiClose() und ssiKill()

ssiClose() - set status to closed, close file descriptors, wait for
             the child to finish (waitpid)
ssiKill()  - free memory, treat list ssiToBeClosed
chg: remove slPrepClose

This reverts the commits 3b238b3 and 67c4640.
chd: redirect ssiClose() to new function ssiCloseList()

This doesn't make much sense in itself, it's only preparation
for closing links by threads.
chg: handle mutex and condition variable for ssiClose correctly
add: ssiCloseList for inter-thread communication
chg: replace omFreeSize() by omFree() for link_lists
chg: globally block SIGUSR1
chg: use a thread for closing ssi links
chg: rewrite sig_term_hdl()

+ use mutex instead of ssiToClosed_inactive
chg: reset ssiCloseList within child after fork()
add: ssiToBeClosed_cleanup()

This function is called before fork() and before termination of the process.
chg: use pthread_atfork()
chg: change some comments
chg: send SIGUSR1 at the end of ssiCloseList_quit()
chg: change error message
chg: in ssiCloseList(), send SIGUSR1 only if list is not NULL
chg: initialize return value in ssiGet_WaitForClose()