Ensure mutt stays in endwin during calls to pipe_msg() (closes #3929)

Ensure mutt stays in endwin during calls to pipe_msg()  (closes #3929)

The previous commit solved the problem reported in #3929: progressbar
output while downloading a message via IMAP was refreshing the mutt
ncurses session after launching the pipe program.

To ensure another place in the code doesn't inadvertantly generate
output, wrap OPTKEEPQUIET around the calls to
pipe_msg()/mutt_wait_filter() too.