when receiving mail for user+tag@ and maildir is setup, we create a .tag

when receiving mail for user+tag@ and maildir is setup, we create a .tag
subdirectory in the maildir, add missing check to strlcat() so that if path
with .tag exceeds SMTPD_MAXPATHLEN we fail instead of creating a .tag dir
that's truncated.

(void) cast strlcpy()/strlcat() that cannot truncate