Implement basic flavors.

Do not start migrating any ports, a hook will prevent it

This has been a long awaiting feature, most of the work has been done by
bapt, bdrewery and antoine, I am just the one actually doing the commit.

All this informations, and more to come are in the first link to our wiki
in the bottom block. A roadmap is in the second link.

To define a different flavors in a port, before any include, set:

FLAVORS= flavor1 flavor2 [...]

The first flavor in the list will be the default.

You can then check for flavors after includ'ing bsd.port.options.mk with:

.if ${FLAVOR} == flavor2
[some stuff]
.endif

To build flavor2, simply run:

make FLAVOR=flavor2

To depend on a specific flavor, write @<flavor> at the end of the depend
string, like:

RUN_DEPENDS= something:origin@foo

Submitted by: bapt, bdrewery, antoine
Reviewed by: portmgr
More infos: https://wiki.freebsd.org/Ports/FlavorsMigration
Todo List: https://wiki.freebsd.org/Ports/FlavorsAndSubPackages
With hat: portmgr
Differential Revision: https://reviews.freebsd.org/D10327