Add distinct potential steps into Makefile.top for putting
sysconf into place, instead of appending the steps for doing so
to the generic POSTINSTALL.
SYSCONFINSTALL is used for 'make install' and unconditionally
copies sys/unix/sysconf to the install directory.
SYSCONFENSURE is used for 'make update' and only copies
sys/unix/sysconf to the installation directory if it doesn't
already exist.
The initial trigger for revisiting this was because of new reports
that cp from (GNU coreutils) 9.4 is now issuing a warning during
NetHack builds. The warning pertains to deprecated use of the '-n'
switch with cp, which is used to only copy the file if the target
does not exist.
After this update, the shell is used to make that determination,
and if the file doesn't exist, cp is now invoked on Linux without
the '-n' switch.
For the macOS hints file, macOS.370, cp wasn't being used for sysconf
anyway, a utility script was being invoked and this doesn't change
that. macOS.370 was updated to remain in sync with linux.370 and
Makefile.top regarding the use of SYSCONFINSTALL and SYSCONFENSURE.
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