generator: Implement BufferIn parameter type (RHBZ#501889).

generator: Implement BufferIn parameter type (RHBZ#501889).

The BufferIn argument turns into various things:

 in C      const char *, size_t parameter pair

 in XDR    an opaque<> type (instead of string) which allows \0 chars

 in other bindings
           mostly just a string, since most languages except for C
           permit strings to contain any 8 bit data