better null characters filtering process

Filtering null characters in hex2char() is not enough, as normal strings
(ie, STRING values in SNMP) often contains such characters also. This
commit change functions behavior to separate concerns:
- hex2char() translates from hexadecimal to ascii
- _getCanonicalString() filters undesirable characters out
  (null-character only currently)

Thanks Vadim Pisarev <v.pisarev@corp.badoo.com> for debugging the issue.

closes: #2784
filter bogus case earlier
initial support for FiberChannel interfaces

see: #2759
move under Networks namespace

see: #2759
cosmetics

kill spurious spaces, and use explicit iterator

see: #2759
make the code testable

see: #2759
use dedicated interface type

see: #2759
move under Networks namespace