makeconf.py: Adds disable and enable functionality

To allow users to momentarily allow portage to "forget" an overlay
dir, the ability to disable and then re-enable an overlay has been
added.

X-Gentoo-Bug: 512316
X-Gentoo-Bug-URL: https://bugs.gentoo.org/512316
reposconf.py: Adds stub disable and enable functions

X-Gentoo-Bug: 512316
X-Gentoo-Bug-URL: https://bugs.gentoo.org/512316
repoconfmanager.py: Adds disable and enable functions

X-Gentoo-Bug: 512316
X-Gentoo-Bug-URL: 512316
db.py: Implements disable and enable functions

X-Gentoo-Bug: 512316
X-Gentoo-Bug-URL: https://bugs.gentoo.org/512316
api.py: Adds disable and enable repo functions

X-Gentoo-Bug: 512316
X-Gentoo-Bug-URL: https://bugs.gentoo.org/512316
cli.py: Adds disable and enable cli functions

X-Gentoo-Bug: 512316
X-Gentoo-Bug-URL: https://bugs.gentoo.org/512316
argsparser.py: Adds --disable and --enable cli args

X-Gentoo-Bug: 512316
X-Gentoo-Bug-URL: https://bugs.gentoo.org/512316