split extra-tools

doc/html/66-envfile.html

doc/html/66-getenv.html

doc/html/66-gnwenv.html

doc/html/66-which.html

doc/html/66-writenv.html

doc/html/execl-cmdline.html

doc/html/execl-envfile.html

doc/html/execl-subuidgid.html

doc/html/index.html

doc/man/66-envfile.1.scd

doc/man/66-getenv.1.scd

doc/man/66-gnwenv.1.scd

doc/man/66-which.1.scd

doc/man/66-writenv.1.scd

doc/man/execl-cmdline.1.scd

doc/man/execl-envfile.1.scd

doc/man/execl-envfile.5.scd

doc/man/execl-subuidgid.1.scd

src/extra-tools/66-envfile.c

src/extra-tools/66-getenv.c