If there is any x86 binary in the chain of parent processes from the current one, then uname -m prints x86_64 and clang defaults to building x86_64 binaries. Detect arm64 using the kernel version and force building an arm64 toolchain instead of perpetuating x86. This is particularly important when the user shell is rc, which has been built for x86, because then rebuilding under rc continues to use x86.

It's sometimes useful to know whether there's been editing activity in a window. This PR adds that information to the ctl file. Change-Id: I21a342ac636dd5c7701b3ed560e3526867329c2c

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>

Fixes: % sam -d -. w foo foo: (new file) #0 w foo ?warning: write might change good version of `foo'

commit 385a6d5877258cee0cac6151e6359c9206006b01 removed src/lib9p/_post.c from the code base, but overlooked removing a reference to the _post.o object file from the src/lib9p/mkfile. This results in lib9p failing to compile: * Running on Darwin... * Compiler version: Apple clang version 12.0.5 (clang-1205.0.22.11) * Building mk... * Building everything (be patient)... >>> mk: don't know how to make '/Users/sasha/plan9port_fork/lib/lib9p.a(_post.o)' in /Users/sasha/plan9port_fork/src/lib9p mk: for i in ... : exit status=exit(1) Remove _post.o from the list of dependent object files from src/lib9p/mkfile to have lib9p compile. Fixes: 385a6d587725 ("lib9p: Remove postmountsrv (#505)")

Also update install.txt, which mistakenly contained intro(1). Pointed out by Nicholas Schwartz.