Don't mention bzip2, gzip in version as they don't actually work
Update file magic for BLAKE2
Ignore ctags file
Fix broken compatibility tests for MD4 mode.

rdiff sig defaults (temporarily) back to MD4, but gains an option to control
this. In MD4, truncate the hash the same way that previous versions did.

Add a sig_magic parameter to rs_sig_begin and rs_sig_file to let callers
choose a version.

Add NEWS about md4 weakness.