Generic FLAVORS work.

• Enable FLAVORS.
• Make make describe flavors aware.
• Add a qa check for unique package names amongst flavors.
• Make MOVEDlint understand flavors.
• Add a bit of sanity check to make sure FLAVORS stay lowercase.
• Various fixes.

Reviewed by: portmgr
Sponsored by: Absolight
Differential Revision: https://reviews.freebsd.org/D12577