Define a new 'struct cheri_sigframe' that will be what we place on signal
stacks to hold the CHERI register frame, but also exception information
such as the CP2 'capcause' register value.
Improve TODO commenting in switch.S: where do we need to voluntarily save
and restore kernel-thread CHERI state in order to properly maintain kernel
CHERI contexts.
Two further spots that require (eventually) saving and restoring kernel
CHERI register state.  This is less pressing than switch.S as it isn't
really needed until we start using CHERI registers throughout the kernel.
Annotate spots where future involuntary kernel CP2 context switching will be
required.
Checkpoint partial support for saving and restoring additional
capability-register state as part of signal delivery and return.

Some groundwork for ucontext system-call support, but only a
little.