x11-toolkits/itk: add LICENSE metadata

The license grants rights to (re)distribute the code/binaries,
so LICENSE_PERMS were adjusted accordingly (w.r.t. the original patch
from PR: it has only "auto-accept" that will make the port and distfile
to disappear from FreeBSD packages/CDN).

While I am here: separate USES/USE_* section from the rest.

PR: 279064
Maintainer timeout: half a year
Approved by: yuri@ (mentor)
Signed-off-by: Eygene Ryabinkin <rea@FreeBSD.org>