Added SILC Rand API, SILC Global Variables API and silcruntime.h.in

Added SILC Rand API, SILC Global Variables API and silcruntime.h.in

The SILC Rand API provides simple PRNG.  The PRNG implementation
uses the Mersenne Twister PRNG.  Added test program for it too.

The SILC Global Variables API provides portable way to add process
global or thread global variables.  On some platforms global
variables cannot be directly used and this API provides portable
way to use them.  Added support for Unix, Windows and Symbian.
Added test program too.

Other changes include the addition of silcruntime.h.in the main
SILC Runtime Toolkit header file.  Added also silc_hash_destructor,
a generic destructor callback for convenience.