rc.d: Generate machine-id from hostid_save

rc.d/hostid_save saves a UUID generated by rc.d/hostid in /etc/hostid.
Store the same UUID, without hyphens, in /etc/machine-id. The hypĥens
are removed with a shell function because hostid_save runs before file
systems are mounted so other tools may not be available yet.

This eliminates some duplication between hostid and machine-id and for
virtual machines machine-id now contains the UUID configured in the
hypervisor like it does on Linux.

Reviewed by: delphij
Discussed with: bapt
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D38811