relocating variable definitions in etc/src/

aling rule: e.g. use_XXX XXX_rules XXX_dir XXX_cache_dir

rename:
$recipient_limit -> $smtp_recipient_limit
$incoming_mail_dir -> $incoming_mail_cache_dir
$outgoing_mail_dir -> $outgoing_mail_cache_dir
$qmail_control_virtualdomains_file -> $qmail_virtualdomains_file

s/header_filter/article_header_filter/
s/header_rewrite/article_header_rewrite/
s/body_filter/article_body_filter/

s/subject_tag/article_subject_tag/

verify_XXX_* -> check_XXX

newly defined:
$use_digest_header_rewrite

removed:
$smtp_log_dir
$smtp_log_file

temp_file_path() ensures temp file name uniqueness by counting the
number of times this function is called.

define terms on "mail" and "message": _message_ -> _mail_
permit_members_only -> permit_member_maps