Fill in the 0.5 release date in the README

I forgot to do this for the actual release. Oops.
Add some TODOs
Add additional legalese
Speed up I/O in batv-validate
Makefile: Add DESTDIR support to 'make install'
batv-keygen: remove unused variable
Clarify legalese
Update documentation to say that IPv6 is supported
Update Debian package instructions: build unsigned package

(since I'm the only one who has the private key to sign the package)
Use _exit, not std::exit, to exit parent after daemonizing

This is rather pedantic, but to ensure a completely transparent
replacement of the parent, we need to use _exit.  Otherwise atexit
handlers and destructors could be called twice: once in the parent and
again in the child.
Add missing #include
Makefile: don't compile with -ansi
Makefile: avoid use of non-standard $^
Fix OpenSSL thread id callback on non-Linux systems

On FreeBSD, pthread_t is not implicitly castable to an unsigned
long (it's a pointer, so we can reinterpret_cast to an unsigned
long).
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