CTL - add variadic arguments API

[KFilipek]

Add variadic arguments API

This issue introduces generic CTL API for the following functions:

umfCtlGet(const char* name, ...);
umfCtlSet(const char* name, ...);
umfCtlexec(const char* name, ...);

Current API contains explicit arguments like:

int umfCtlGet(const char *name, void *ctx, void *arg);