For cross-compiling plan9 from Unix, provide a way
to force `mk` to use `rc` instead of `sh` without
setting `MKSHELL` in individual `mkfile`s.

If the environment variable `FORCERCFORMK` is set,
`mk` will default to using `rc`, not `sh`.

Signed-off-by: Dan Cross <cross@gajendra.net>

On March 23, 2021, Nokia transferred the copyrights in the Plan 9 software
to the Plan 9 Foundation, which relicensed them under the MIT license.

This commit updates the Plan 9 from User Space license to reflect the
new base license. The vast majority of the contributions beyond the
base Plan 9 set were by me, many of them explicitly under an MIT license.
Those are all under the new MIT license now as well.

The port of mk to Unix was taken from Inferno via Vita Nuova and had
been made available under GPL, but Vita Nuova has relicensed Inferno
under the MIT license as well, to match the new Plan 9 license.

Michael Teichgraber contributed src/lib9/zoneinfo.c explicitly under
the Lucent Public License but has agreed to change the contribution
to the MIT license now used in the rest of the distribution.

There remain a few exceptions, most notably fonts.
See the root LICENSE file for full details.

The only mention of the Lucent Public License in the whole tree now
is in the LICENSE file, explaining the history.

Found by ASAN.

Avoid signed integer overflow using ulong instead of long h.

This brings mk's behavior when using rc in line with Plan 9's.
The existing code is for Unix environment data structures but
also was assuming Unix shell semantics where empty and missing
variables are mostly equivalent.

The Plan 9 code (/sys/src/cmd/mk/plan9.c in the distribution)
explicitly removes /env/name (creating an empty list) when the
value is missing or an empty string.

Fixes #255.

As usual, gcc finds some real problems but also reports a ton
of noise. Fix the problems and quiet the noise.

Remote whitespace at the ends of lines.
Remove blank lines from the ends of files.
Change modes on source files so that they
are not executable.

Signed-off-by: Dan Cross <cross@gajendra.net>

In general, no space after `if` etc, and no
braces for a single statement inside of a loop
or conditional.

Signed-off-by: Dan Cross <cross@gajendra.net>