Allow sharing of common code between different hints files
through use of: #-INCLUDE
new folder created: sys/unix/hints/include
new hints include files:
sys/unix/hints/include/multiw-1.2020
sys/unix/hints/include/multiw-2.2020
structure the early parts of sys/unix/hints/linux.2020 and
sys/unix/hints/macOS.2020 consistently, and utilize #-INCLUDE multiw-1.2020
and #-INCLUDE multiw-2.2020 in them. That will allow the Makefile lines
that they contain to be maintained in a single place.
bdbf8f19cc