Rewrite enc_mode_inttostr() and enc_mode_strtoint().

Make sure both functions works the same way and refer to the same
encryption mode string.

Updated the fwknop usage message to display the encryption mode.
Added tests to validate the encryption mode for the client.
Renamed the CBC legacy VI encryption mode by legacy as mentionned in the man page.
Merge remote-tracking branch 'fjoncourt/master'

Merged update from Franck - closes issue #71.
minor write_test_file() path bug fix