bsdinstall: use the canonical name for compression

According to zfsprops(7), the canonical property for the compression
algorithm is "compression", with "compress" accepted as an alternate
name. Use the canonical name in bsdinstall.

While here, change "lz4" to "on" so we pick up any future changes in
the default compression algorithm.

MFC after: 1 week
Reviewed by: delphij
Differential Revision: https://reviews.freebsd.org/D51572

(cherry picked from commit 16045420e7f83489ecd5e2163aa9bb11236962f5)