partedit: make minimum FreeBSD install size just under 1G

[emaste]

Summary

1G is a convenient disk image size for testing.  It is also the
installer's minimum size, but the minimum applies to the partition not
the whole disk.  When testing with a 1G image this resulted in the
counterintuitive error "There is not enough free space on <disk> to
install FreeBSD (1.0 GB free, 1.0 GB required)."

Reduce the installer's minimum required by 1M to allow this case.

Sponsored by:   The FreeBSD Foundation

[emaste]

{F57270726}