ynl-gen-cpp: add support for generating nl80211

[zb-gong]

Fixed some errors when generating nl80211:

1. AttrSet did not support when sub-type of indexed-array is u32.
2. scalars did not include types of s8 and s16.
3. Type did not properly initiate c_name when the first character is a digit.
4. For TypeArrayNest, we do not need a member to count the number of elements in C++.