minor typo fix
[server] added --pcap-any-direction along with config file support

From the config file comments:

This variable controls whether fwknopd is permitted to sniff SPA packets
regardless of whether they are received on the sniffing interface or sent
from the sniffing interface.  In the later case, this can be useful to have
fwknopd sniff SPA packets that are forwarded through a system and destined
for a different network.  If the sniffing interface is the egress interface
for such packets, then this variable will need to be set to "Y" in order for
fwknopd to see them.  The default is "N" so that fwknopd only looks for SPA
packets that are received on the sniffin

PCAP_ANY_DIRECTION         N;
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.