Fix output value of sd_seat_get_sessions() and drop FOREACH_WORD use

sd_seat_get_sessions() would return 0 in the 'n_uids' (now 'ret_n_uids') output
parameter when 'uid' (now 'ret_uids') was passed as NULL.

While at it, drop FOREACH_WORD() use.

Also use any whitespace as separator. In practice this shouldn't matter, since
logind always uses spaces, but it seems nicer to not specify this explicitly,
and the default is more flexible.