Reorganize the command execution code to separate out the pty and

Reorganize the command execution code to separate out the pty and
non-pty code paths into their own event loops.  The non-pty exec
code is now contained in exec_nopty.c and the pty exec code is split
between exec_pty.c (parent process) and exec_monitor.c (session leader).
This results in a small bit of duplicated code but improves readability.
Some of the duplicated code will fall out in future changes to the
event subsystem (the signal pipe).