use.desc: Introduce global USE=magic

use.desc: Introduce global USE=magic

USE=magic is currently used consistently by 12 packages:

app-arch/engrampa[magic] Enable filetype auto-detection via
  sys-apps/file
app-editors/nano[magic] Add magic file support (sys-apps/file) to
  automatically detect appropriate syntax highlighting
app-misc/vifm[magic] Use libmagic to determine mimetypes
app-misc/worker[magic] Add magic file support from sys-apps/file to
  automatically detect file types
app-text/zathura[magic] Use libmagic to determine mimetypes
media-gfx/qiv[magic] Use libmagic to determine mimetypes
media-libs/libextractor[magic] Enable magic support using sys-apps/file
media-sound/moc[magic] Use libmagic to determine mimetypes
net-misc/gerbera[magic] Use libmagic to determine file types
net-p2p/mldonkey[magic] enable use of libmagic
sci-geosciences/viking[magic] Use libmagic to determine mimetypes
www-servers/pshs[magic] Enable automatic detection of Content-Type using
  libmagic (sys-apps/file)

Signed-off-by: Michał Górny <mgorny@gentoo.org>