lib9: fix runcmd, removeall, and tempdir functions

cmd/dist: Include run_plan9.c and tempdir_plan9.c
        from lib9 for build, and in general consider
        file names containing "plan9" for building.

cmd/ld: provide function args for the new functions
        from lib9.

R=rsc, rminnich, ality, bradfitz
CC=golang-dev
https://golang.org/cl/7666043