Cosmetic changes to netd client files.

+ Name the dispatch header correctly (NetdClientDispatch.h).
+ Hide the global dispatch variable (__netdClientDispatch).
+ Explain why it's okay to read the variable without locking.
+ Use quotes instead of angle-brackets for non-system includes.
+ Add necessary declarations for C compiles (and not just C++).

Change-Id: Id0932165e71d81da5fce77a684f40c2263f58e61