Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

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

If the special target .MAKEFILEDEPS exists, then enable the
"remaking makefiles" feature. Otherwise, follow traditional Pmake behavior.

Add a .sinclude directive which does the exact same as .include, except
whine when the file cannot be found and opened.

Introduce a new pseudo-target .EXPORTVAR which allows to put a
make macro into the environment of programs executed by make. This
has approximately the same function as gmake's export directive.
The form of a pseudo target was deliberately choosen to minimize work
for POSIX compatibility (Makefiles are not allowed to use any targets
starting with a dot and consisting only of uppercase letters except those
specified in the standard when they want POSIX compatible behaviour, so
such a Makefile can never contain .EXPORTVAR.)
Change the handling of macros coming from the environment: instead
of asking the environment for each variable we could not find otherwise
put all the environment variables in a special variable environment just
at start up.

This has been tested on the ports cluster by kris.

Submitted by: Max Okumoto <okumoto@ucsd.edu>

Rename the directive_hash.[ch] files to hash_tables.[ch] now
that there are more than one hash table in them. There is no
history to preserve here, so go without a repo-copy.

Asked for by: Max Okumoto <okumoto@ucsd.edu>