server: Kill temporary SIGCHLD handler.

server: Kill temporary SIGCHLD handler.

During startup, para_server sets up a temporary signal handler for
SIGCHLD in order to notice if the afs child process terminates
immediately (doing the signal setup after the fork would be too
late). This patch makes it block this signal during this period
instead. This works equally well and it fixes a weird problem on Mac
OS where the temporary SIGCHLD handler was executed although it had
been replaced by the generic signal handler of signal.c.