* Add USE_GITLAB support for downloading from gitlab sites
GitLab is widely used and it is long due for added support to simplify.
This implementation has a similar format as the USE_GITHUB and should
be easy to adopt.
Sample:
```USE_GITLAB= yes | nodefault
GL_SITE= protocol://domain:443 (https://gitlab.com)
GL_ACCOUNT= accountname (${PORTNAME})
GL_PROJECT= projectname (${PORTNAME})
GL_COMMIT= 40charactercommithash (None, required variable)
GL_TUPLE= [site[:port][/webroot]:]account:project:commit[:group][/subdir]
https://foobar.com:9838/gitlab:accountname:projectname:160bitcommithash:group/subdir```
Reviewed by\: lifanov (mentor), matthew (mentor)
Approved by\: lifanov (mentor), matthew (mentor)
Differential Revision\: https://reviews.freebsd.org/DXXXXX