pmm: Use 'struct segoff_s' in pmm header.

pmm: Use 'struct segoff_s' in pmm header.

Minor cleanup - use 'struct segoff_s' instead of a manual
segment/offset pair.

Also, while at it, define the header signature at compile time instead
of at run-time.  (Defining at run-time is a hold over from when
variable changes were persistent across soft-reboots.)

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>