terminal: Restore SIGPIPE signal handler to oldact (SIG_DFL)

Prior to executing a new command restore the parent process to the
default SIG_DLF handler as we normally use SIG_IGN it to avoid
crashing on pasted input.

As we seem to be hitting a Broken Pipe message when running
certain scripts, it seem we need to restore back the
default handler.

This is similar to what other folks have been doing in
https://github.com/labwc/labwc/issues/1209

.

Fixes: #994

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>