bsdinstall: Emit a warning if the system has too little memory

PR: 251993
Reviewed by: adrian
Event: Kitchener-Waterloo Hackathon 202506
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D50944

(cherry picked from commit eb5884c564ae0e37df1d8a3fc5704ab6b2fa07d0)