Change iniparser_getint64(...) to iniparset_getlongint(...) and add unittest for iniparser_getlongint(...) and also add interface for longint equal in CuTest.h/CuTest.c.

The documentation disclaims support for building with a C++ compiler, so
it is reasonable to assume that the library will be built with a plain C
compiler, so the functions will all have plain C linkage.  By default, a
C++ application that wished to use libiniparser would need to wrap the
inclusion of libiniparser headers in 'extern "C" { ... }' to reflect the
C linkage of libiniparser.  Instead, place that marker directly in the
libiniparser headers, so that client applications do not need to care.
This has no effect on normal compilation of libiniparser, since the new
markers are inside a '#ifdef __cplusplus' guard, and straight C
compilers do not define __cplusplus.