geom(4): Kill GEOM_PART_EBR_COMPAT option

Take advantage of Warner's nice new real GEOM aliasing system and use it for
aliased partition names that actually work.

Our canonical EBR partition name is the weird, not-default-on-x86-prior-to-
this-revision "da1p4+00001234." However, if compatibility mode (tunable
kern.geom.part.ebr.compat_aliases) is enabled (1, default), we continue to
provide the alias names like "da1p5" in addition to the weird canonical
names.

Naming partition providers was just one aspect of the COMPAT knob; in
addition it limited mutability, in part because it did not preserve existing
EBR header content aside from that of LBA 0. This change saves the EBR
header for LBA 0, as well as for every EBR partition encountered. That way,
when we write out the EBR partition table on modification, we can restore
any bootloader or other metadata in both LBA0 (the first data-containing EBR
may start after 0) as well as every logical EBR we read from the disk, and
only update the geometry metadata and linked list pointers that describe the
actual partitioning.

(This change does not add support for the 'bootcode' verb to EBR.)

PR: 232463
Reported by: Manish Jain <bourne.identity AT hotmail.com>
Discussed with: ae (no objection)
Relnotes: maybe
Differential Revision: https://reviews.freebsd.org/D24939