logind: rename "seat-master" tag to "master-of-seat"

We currently enforce that seats are to be named in the form of
"seatXXX", i.e. need to begin with the 4 characters "seat". Thus,
"seat-master" would qualify as a seat name. As seat names are frequently
used as tags on devices, the "seat-master" tag might hence confuse
logind if the user decides to name a seat "seat-master".

Hence, avoid any ambuigity: let's rename the "seat-master" tag to
"master-of-seat".