Better registry support

[justanotheranonymoususer]

I skimmed over the registry functions and I have several suggestions:

• Several useful options are missing. For example RegGetValue receives no options at all, but:
  • RRF_SUBKEY_WOW6432KEY/RRF_SUBKEY_WOW6464KEY are necessary sometimes (with this function and RegOpen*/RegCreate* too)
  • RRF_NOEXPAND is always used, why?
  • Restricting by type can be useful, e.g. if I only expect DWORD, let me pass RRF_RT_REG_DWORD for performance and ease of use
• Buffer size handling can be improved
  • This part is kinda weird: if data_len1 == data_len2 + 2 { // https://stackoverflow.com/q/29223180
  • As in our previous discussion, just use the buffer you got with the data if its <= the initial buffer
  • If new_len > initial_len, the content might have changed, probably better to retry than to fail
• Data validation is very simplistic (validate_retrieved_reg_val)
  • REG_SZ is checked for null terminator, but what about REG_EXPAND_SZ? REG_MULTI_SZ?
  • As far as I understand, parse_multi_z_str is unsafe because it's not bounded by the size but the buffer can be without the double null terminator