seq: update_group_ports - rewrite blknames update

seq: update_group_ports - rewrite blknames update

Resolve the warning bellow and use more optimized calls.

seqmid.c: In function ‘update_group_ports’:
seqmid.c:672:45: warning: ‘%s’ directive output may be truncated writing up to 127 bytes into a region of size 61 [-Wformat-truncation=]
  672 |                                          ", %s", bp->name);
      |                                             ^~
seqmid.c:671:33: note: ‘snprintf’ output between 3 and 130 bytes into a destination of size 63
  671 |                                 snprintf(blknames + len, sizeof(blknames) - len,
      |                                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  672 |                                          ", %s", bp->name);
      |                                          ~~~~~~~~~~~~~~~~~

Signed-off-by: Jaroslav Kysela <perex@perex.cz>