net/unison*, audio/asterisk-*: Fix syntax errors in how FLAVOR is set

[tobik]

Summary

Something like ${FLAVORS[1]} looks up a variable named FLAVORS[1].
It is not equivalent to ${FLAVORS:[1]} which looks up the first
word of a variable named FLAVORS. The second form is what should
be used to set FLAVOR as otherwise it is set to an empty value
and we might as well replace FLAVOR?=${FLAVORS[1]} with FLAVOR?=.
This potentially breaks ports builds of the default flavor outside
of Poudriere since something like

FLAVORS=	foo bar
FLAVOR?=	${FLAVORS[1]}
.if ${FLAVOR} == foo
# ...
.else
# ...
.endif

will *always* take the else branch.

In the asterisk-* ports we do not really need to set FLAVOR but
I left it in anyway.

Test Plan

Poison ports with

.if empty(FLAVOR)
.error whoops
.endif

directly after FLAVOR?=. It should produce an error before
applying this change but not afterwards.

[madpilot]

Thanks for noticing this. It clearly was an oversight on my part!