guix-install.sh: Create an init profile during installation

guix-install.sh: Create an init profile during installation

Create an init profile (/etc/profile.d/guix.sh) during installation for
better integration with foreign distros. This file, read by login-shells
sets a few environment variables (PATH, GUIX_PROFILE, GUIX_LOCALE, along
with XDG_DATA_DIRS) and makes guix-installed packages readily available
for the users. Other environment variables, as listed by `guix package
--search-paths=prefix`, are also exported.

Checks are in place to prevent the init profile from needlessly polluting
the user's environment.

* etc/guix-install.sh (sys_create_init_profile): New function to create
/etc/profile.d/guix.sh init profile.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>