This revisits the change made in da0fe745fc:
1) Changes c-macro constants back to their original definition (without
explicit typing them as unsigned) so-as to more exactly compare
the comedilib version of comedi.h with that of the kernel module.
2) Removes the duplicate definition of NI_GPCT_INVERT_CLOCK_SRC_BIT in
comedi.i.
3) Instead of redefining the macros to fix the mishandling of signedness
by SWIG for python, teach SWIG to treat these as unsigned ints.