mail/spampd: Update to 2.51 and refine rc script

• Include /usr/local/bin and /usr/local/sbin to PATH (this way pyzor and razor work, which wouldn't be used otherwise [debug output would show that they're silently skipped])
• support for sockets
• IPv6 support
• Fix injected X-Envelope-* headers so that spamassassin actually can pick them up for SPF and other checks
• support for sid (so patch-setsid not required anymore), toggled by --setsid (rc script always sets this so it doesn't break on update)
• new rc var (spampd_extra_flags), so it's easy to use the default setup and just add deltas in rc.conf like in:
  
  spampd_enable="YES" spampd_extra_flags="--sef"

PR: 227903
Approved by: maintainer timeout