Remove already done TODO

Just see the patch if you're not believing me.
Rationalize checking the volume's state

We used to check for the BitLocker volume state only when receiving a
decryption request and printed a warning/did checks each time.
We now check the volume state upon receiving an encryption or decryption
request, and the volume state check is done only once and remembered for
the following checks.
Put the default dislocker into a variable

The default dislocker was done upon installation using ln -s on
$(BIN)-fuse, which is not good if dislocker-fuse wasn't compiled.
Now, the default dislocker points to $(DEFAULT_PROG), which is the first
binary in the BINS list.
The idea is to make possible to compile and install only on binary like
so for instance:
 make dislocker-file
 make install BINS=dislocker-file
Change some notes

Add a note for compiling and installing binaries independently, now that
it's possible.
Add @Thalience in the thanks, if that's ok.