Remove some totally redundant code
Improve error handling when loading key files

In particular, reject empty keys
makefile: install batv-keygen
batv-validate: improve usage message
batv-sendmail: improve a comment
batv-sendmail: support $BATV_KEY_MAP_FILE
Add man pages
README: use https URLs for mailing lists
milter: allow UNIX socket paths to be prefixed with "unix:" or "local:"

This is standard milter syntax. Previously the special behavior for
UNIX domain sockets was only executed if the socket path was specified
without this prefix.
Document non-UNIX sockets
Update TODO file
Release 0.6