Fix tcsh losing history when tcsh terminates because the pty beneath it

is closed.

Diagnosed by Ted Anderson:

New signal queuing logic was introduced in 6.15 and allows the signal handlers
to be run explicitly by calling handle_pending_signals, instead of
immediately when the signal is delivered.  This function is called at
various places, typically when receiving a EINTR from a slow system call
such as read or write.  In the pty exit case, it was called from xwrite,
called from flush, while printing the "exit" message after receiving EOF
when reading from the pty (note that the read did not return EINTR but
zero bytes, indicating EOF).  The SIGHUP handler, phup(), called
rechist, which opened the history file and began writing the merged
history to it.  This process invoked flush recursively to actually write
the data.  In this case, however, the flush noticed it was being called
recursively and decided fail by calling stderror.

My conclusion was that the signal was being handled at a bad time.  But
whether to fix flush not to care about the recursive call, or to handle
the signal some other time and when to handle it, was unclear to me.
However, by adding an extra call to handle_pending_signals, just after
process() returns to main(), I was able to avoid the truncated history
after network outages and similar failures.  I verified this fix in
version 6.17.

Approved by:	ed (mentor)
MFC after:	1 week

contrib/tcsh/sh.c

@@ -1291,6 +1291,8 @@ main(int argc, char **argv)
     /*
      * Mop-up.
      */
+    /* Take care of these (especially HUP) here instead of inside flush. */
+    handle_pending_signals();
     if (intty) {
 	if (loginsh) {
 	    xprintf("logout\n");