Extract PLIST_SUB substitutions into a sed script.
This fixes PLIST_SUB being too big and PLIST_SUB_SED getting bigger than
_POSIX2_LINE_MAX.
PR: 222355
Reported by: asomers
Reviewed by: asomers, mfechner
Sponsored by: Absolight
Differential Revision: https://reviews.freebsd.org/D14014