A modest proposal: let's add PID to assertion failure messages

In these days when we run almost all test cases in parallel, it's
frequently not that easy to tie a "TRAP: ..." message in the log
to nearby log messages.  (The postmaster's subsequent complaint
often helps, but it could be some distance away in the log; and
good luck untangling things if more than one Assert failure happens
concurrently.)  We could add a simple bread crumb trail by
including the process's PID in such messages.  Any objections?

			regards, tom lane