bsdinstall: Use a login shell for final configuration.

If the user accepts our offer of a shell to perform final configuration
tasks before rebooting, start a login shell. This ensures it will have
the correct PATH and be able to install packages without issues.

PR: 286722
MFC after: 3 days
Reviewed by: jrtc27, allanjude, emaste
Differential Revision: https://reviews.freebsd.org/D50297

(cherry picked from commit 5870d6a1ef4ec5d6fc91d0f1635bca59e09a040d)