zz_main: unset GREP_OPTIONS to avoid confusions
systemd: add some shortcuts for :systemdctl

They are: status, stop, start commands
Makefile will clean :systemd utils
00_core: add :export function, which is :noop
Functions can be marked "exportable" from lib/*.sh file

When developing a library, we can mark some function as *exportable*.
By putting ":export symbol _internal_function _and_arguments" in the
library file, the "bin/compile.sh" script will know and generate a
case statement to check if user input program (name) is valid. This
statement was written manually in "lib/zz_main.sh". The syntax is
quite strict. "_internal_function _and_arguments" should not contain
any quotation mark(s). Example

:export s-makepkg _makepkg  # see lib/pkgbuild.sh
:export start _systemd_ctl start # see lib/systemd.sh

Woh. This is quite tricky way, but it makes (hacker) life easier!

Please note that, each exportable function will be a symbolic link.
In our examples, we have

/usr/bin/s-makepkg -> /usr/bin/s-run
/usr/bin/start -> /usr/bin/s-run

These links are created/removed automatically by Makefile.

FIXME: Makefile may not work correctly when PREFIX is specified.
pkgbuild.sh: export s-makepkg function
pkgimport.sh: export s-import-package function
systemd.sh: export status, stop and start functions