Improve NetworkIncrement

+ Set 1 as default value for increment.
 + Update IPsecUtil.
+ Tolerate address with host bits set when incrementing.
 + Call sites can check initial value on their own.
+ Support multiple ways of converting to string.
 - Only the previous "dash" format is supported here.
+ Update docstrings.

Change-Id: I0c71a6327cca6a319715b3fcfbbee800cac14287
Signed-off-by: Vratko Polak <vrpolak@cisco.com>