Add child stats for process name, SA RULESVERSION for logging, and some optimizations.

[mpaperno]

• Add detection and logging of "RULESVERSION" tag with SA >= v3.4.0. (#32)
• Add tracking of some per-child runtime statistics.
• Add ability to provide a custom child process name template string (or not modify the child name at all). Template format documented in POD. (#32)
• Optimize initial header processing when building message line array in process_message().
• Slight optimization to LMTP multi-recipient handling in process_request().
• Optimize how rewritten (tagged) message is saved back to temp file: just write all the contents at once instead of line array loop; Any added X-Envelope-To header is removed with regex.
• Add (inject) _SPAMPDVERSION_ as a "template tag" (macro) before rewriting the message with SA (eg. for use in add_header directives). (https://github.com/mpaperno/spampd/issues/32#issuecomment-890486418 again :-)

Closes #32