fork() refactoring

This patch moves all the common code that is usually invoked before
doing a fork() into a single function, fork_process(). It is not aware
of the EXEC_BACKEND machinery, so it should be used as fork() currently
is -- inside an #ifndef EXEC_BACKEND block, if appropriate.

I wasn't sure whether to put this under backend/port, postmaster.c, or
in a separate .c file. This patch places the code in a new file
postmaster/fork_process.c -- suggestions on whether this is the right
location would be welcome.

Barring any objections, I'll apply this to HEAD on Monday.

-Neil