bsdinstall: allow whitelabeling the installer

Override OSNAME to change the name of the OS in the installer.

This is a first step, the shell changes will be separate.

Reviewed by: allanjude
Approved by: allanjude
Differential Revision: https://reviews.freebsd.org/D34878
Sponsored by: Rubicon Communications, LLC ("Netgate")