udev: allow removing tags via TAG-="foobar"

This extends the udev parser to support OP_REMOVE (-=) and adds support
for TAG-= to remove previously set tags. We don't fail if the tag didn't
exist.

This is pretty handy if we ship default rules for seat-assignments and
users want to exclude specific devices from that. They can easily add
rules that drop any automatically added "seat" tags again.

Signed-off-by: Anthony G. Basile <blueness@gentoo.org>