bsdinstall: stop messing with file descriptors

Throughout the bsdinstall script fd 3 is used by f_dprintf (set through
$TERMINAL_STDOUT_PASSTHRU). By closing file descriptor 3 here, the
final f_dprintf "Installation Completed ... does not work anymore.

By putting the code into a subshell, file descriptors can be edited
without interference with the calling script.

Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D35113
Sponsored by: Beckhoff Automation GmbH & Co. KG
MFC after: 3 days

(cherry picked from commit 1f7746d81f53447ac15cc99395bb714d4dd0a4da)