signal: Switch from signal() to sigaction.

signal: Switch from signal() to sigaction.

Use of signal() should be avoided because the behavior of signal() varies
across Unix versions, and has also varied historically across different
versions of Linux.

This patch rewrites para_install_sighandler so that it calls sigaction()
instead of signal(). The implementation is taken from good old APUE.

There are a couple of other users of signal() in the paraslash code. Most
of which are OK because they use signal() only to ignore/reset a signal which
happens to be the only portable use of signal(). All other users of signal()
have to be converted in subsequent patches.