libfgsb3c interface from C

This is the fork of the libfgsb3 from cran with the following differences:

• The return type has changed is is very similar to what optim returns
• Allows a direct C/C++ interface through a R registered function, similar to C interface to optim with 2 additional arguments.
• Allows adjustment of tolerances for minimization success.
• Added xtolAtol and xtolRtol minimization success criterion.
• Added maxit termination