src/vipw.c: Restore the original terminal pgrp after editing

src/vipw.c: Restore the original terminal pgrp after editing

This fixes a problem when the shell is not in monitor mode (job control
enabled) which resulted in the terminal pgrp being set to an invalid
value once vipw exited.

Fixes: 7eca1112fbd7 (2019-11-11; "Fix vipw not resuming correctly when suspended")
Closes: <https://github.com/shadow-maint/shadow/issues/1194>
Reported-by: <https://github.com/yonecle>
Tested-by: Alejandro Colomar <alx@kernel.org>
Reviewed-by: Alejandro Colomar <alx@kernel.org>