Add Common_config class

Use it for commonality between batv-validate and batv-milter.
Add 'X-Batv-Status: invalid' header if a bounce is missing a BATV signature

batv-milter now has primitive bounce detection logic (anything with a null
envelope sender is considered a bounce).  If a bounce destined to a user
with a BATV key lacks a BATV signature, batv-milter adds 'X-Batv-Status: invalid,
missing'.

The X-Batv-Status header has been extended to specify the reason for invalidity.
Valid header values are:
	- valid
	- invalid, missing
	- invalid, bad-signature
	- invalid, multiple-rcpt

'batv-filter -f' also now adds a 'X-Batv-Status: invalid, missing' header
if message did not contain a BATV signature.
Add new milter option on-invalid

It specifies what the milter should do with invalid
bounces ('accept', 'reject', or 'tempfail').
Update NEWS file with recent changes