Create a separate script to generate osreldate.h rather than sourcing

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