RFR: Port of FreeBSD *context routines to DFly

[Matthew Dillon]

    Oh. I am going to commit the mcontext structural changes right now.
    In fact, since I have already ripped up the signal context, I am 
    going to make the same changes to struct sigcontext.

    That is, adding the mc_len, mc_fpformat, and mc_ownedfp fields.

    This also properly aligns the FP context in the structure and will
    allow better handling of the floating point state.

					-Matt
					Matthew Dillon 
					<dillon at backplane.com>