ourselves with a pipe if the plugin will actually log the data.
This avoids a problem with non-interactive commands where no tty
is present where sudo will consume stdin even when log_input is not
enabled in sudoers.
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).
