Add cfutimer which uses more accurate functions if available

This is some code I had previously written, adapted to (mostly)
libcfu coding style. The API is very similar to that of the
existing (to be eventually removed) cfutime.
Properly declare parameterless functions as such

Also use void in the definitions for consistency.
Remove redundant use of extern storage class specifier
Remove old cfutime code
Fixup a few stricter warnings