Make Matlab/Octave's svmpredict understand info().

Currently svmpredict.c calls vm_set_print_string_function uselessly, since the
only variable this function changes, svm_print_string, is not used anywhere in
svmpredict.c, nor is the function info() used. Instead, prints are hardcoded to
be mexPrintf. This causes an issue when one asks for validation and -q, whereby
the -q is ignored, and output is still printed.

This change makes info() in svm.cpp public via svm.h, renames it to svm_info
for consistency, and makes svmpredict.c use svm_info for its validation output.

Let me know if there's anything to change in this patch, or if there's a much better solution :)