Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

MFC r255775 r255796 r255807 r255930 r255929 r255957:

Create a separate script to generate osreldate.h rather than sourcing
newvers.sh into a temporary subshell with inline make rules.
Fixes PR 160646.

Allow the path to the system source directory to be passed in to
newvers.sh. Pass it in from include/Makefile. If it isn't passed in,
fall back to the old logic of using dirname $0. Fixes PR 174422.

PR: 160646 174422

Create a separate script to generate osreldate.h rather than sourcing
newvers.sh into a temporary subshell with inline make rules.

Using a separate script fixes a variety of problems, including establishing
the correct dependencies in the makefiles. It also eliminates a problem
with the way newvers.sh uses `realpath $0`, because $0 expands differently
within a script sourced into a rule in a makefile depending on the version
of make and of /bin/sh being used. The latter can cause build breakage in a
cross-build environment, and can also make it difficult to compile 10.0 on
older pre-10.0 systems.

PR: 160646 174422
Submitted by: Garrett Cooper <yaneurabeya@gmail.com>
Approved by: re (gjb)
MFC after: 2 weeks