Add "save history" command

PR cli/23664 points out that it would sometimes be convenient to
immediately save the current history to a file.  This patch implements
this feature.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=23664
Reviewed-By: Eli Zaretskii <eliz@gnu.org>
Reviewed-By: Keith Seitz <keiths@redhat.com>