Add eselect gitlabhq ebuild

To simplify the steps needed to upgrade to a new gitlabhq slot. I like to introduce an eselect gitlabhq ebuild

I created this ebuild already (will push it soon). It supports the standard eselect syntax, and also a unset.

Additional before it set the new versions it checks:

• emerge --config has been run (assets dir exist)
• if there are still ._cfg files in the config dir This to prevent switch to a broken version

It creates the following links:

• /opt/gitlabhq => gitlabhq-SLOT
• /etc/init.d/gitlabhq => gitlabhq-SLOT (optional, because not needed with systemd) Will also later add an optional systemd hardlink:
• /usr/lib/systemd/system/gitlabhq.target gitlabhq-SLOT.target

Then the config files/ebuilds can use these links, instead paths with the slot, reducing the places where SLOT is needed.