Rename SAVE_CHERI_CONTEXT() and RESTORE_CHERI_CONTEXT() used to save and
restore CHERI user register frames during involuntary traps (and fork
trampolining) to SAVE_U_PCB_CHERIFRAME() and RESTORE_U_PCB_CHERIFRAME(),
which make it more clear that they are saving state to the user PCB (vs.
kernel PCB state).
Define a new structure 'cheri_kframe', which will hold voluntary
context-switch CHERI register state for kernel threads.  This is
a first step towards more maturely maintaining capability-register
state for kernel threads, required so that the kernel can more
cleanly interact with user-originated capabilities.

For now, just define a 'micro-ABI' in which $c11 and $c12 can be
used freely by the kernel without disabling interrupts (unlike
currently), but without a fully preserved register file -- which
will need to be changed once we start using a CHERI-aware compiler
for the kernel.