Don't save the prt_scyl/prt_ecyl values, change them, use them

Don't save the prt_scyl/prt_ecyl values, change them, use them
and then restore them. Just change/use the saved values and skip
the restoring.

Allows PRT_make() to add 'const' to its struct mbr parameter, and
thus allows MBR_make() to add 'const' to its struct mbr
parameter.

No intentional functional change.