Fix various problems detected by Clang 3.4.

- cidr.cpp: remove inverted_bits; unused since 9fad3ecb9215a0034bf407f192926b04cb5efaed.
- cmd_stats.cpp: remove needless inversion and comparison to 0.
- inspircd.cpp: remove needless comparison to 0.
- socket.cpp: remove all_zero; unused since 03a1bf15b1da7643b237c22db1a478916a976ccf.