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.
[server] minor memory leak bug fix during access.conf parsing found by Coverity
[server] minor cosmetic (unnecessary NULL checks and one un-triggerable memory leak) found by Coverity
[server] fix pointer NULL check after strdup() - found by Coverity
[server] varargs cleanup bug fix found by Coverity
[server] minor memory leak bug fix during access.conf parsing found by Coverity
[server] minor memory leak bug fix during SPA digest calculation found by Coverity
Merge remote-tracking branch 'fjoncourt/master'

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