Improve header files: include guards and C++ support + cleanups

Hi Ronnie,

Please pull from github:aredlich/libnfs.git master .

It consists of the following changes:
* add include guards to headers (where missing)
* add 'extern "C"' wrappers to headers to allow using libnfs from C++
* rename variables called "export" in libnfs-raw.h as that conflicts with a C++ keyword
* address clang compiler warnings about the usage of assignment results in while loop conditions
.

Thanks,
Arne