Use 550 code for unavailable actions

For actions that are either unimplemented or unavailable, send 550
instead of any other error code (eg. 450). Otherwise, some FTP clients
like Google Chrome appear to be broken.
Rudimentary UTF8 support

Advertise UTF8 support so FTP clients can interpret non-English file
names properly. Whether or not UTF8 is really supported is up to
implementation module.
Make UTF8 support switchable

A new configuration parameter, 'utf8', controls whether Bifrost-based
FTP server will advertise UTF8 support. It is up to implementation now
to decide whether it should be enabled or not.
Fix FTP/SSL (FTPS) support

FTP/SSL support was broken (at least on R15B) due to socket ownership
problem. There was also a typo in option name in the documentation, and
FTPS mode implemented was incorrectly referred to as "implicit".
Fixed tests

Fixed an overlooked regression in testsuite introduced with 450 -> 550
change.
Merge pull request #11 from vsinitsyn/master

UTF-8 support and being Google Chrome-friendly