seqmid: restore sequencer address prefix match feature

seqmid: restore sequencer address prefix match feature

Commit 19892334499e (seq: Fix for snd_seq_parse_address()) removed the
ability to match sequencer client names by any of by their prefixes in
an attempt to avoid wrong matches when one client name is the prefix of
another.

However, the prefix match feature was documented and actually used.

Allow prefixes to match, but only if there is no exact match.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>