illumos gate

This is another part of the 'IPD 38 Signal Handling, Extended FPU State, ucontexts, x86, and You' work. In particular, this provides a set of three new utilities that provide another way to get at and allocate a ucontext_t so we can include additional state for additional components. This adds a trio of three functions here:

• ucontext_alloc() which will allocate a new ucontext_t
• ucontext_free() which frees a previously allocated one that was created with this
• getcontext_extd() which changes the assumptions around initialization.

For more background, see the IPD.