Doc patch to perlipc

Doc patch to perlipc

1. I've removed some erroneous code regarding signal names (it doesn't
handle signal aliases) with a reference to a module that does handle
it correctly.
2. Removed the sample on temporary signal ignoring; this is not likely
to do what users want it to do. Possibly this should be replaced by a
discussion on sigprocmask & friends, but perhaps we can skip in
altogether
3. Corrected the discussion on permissions to take the difference
between real, effective and saved UIDs into account.
4. Removed all mention of the SysV signaling issue. This was rather
relevant when perlipc was written 15 years ago but it isn't anymore
nowadays.
5. Removed the suggestion that you can longjmp out of a signal handler
(see CERT's SIG32-C).