rename FPBASE to _FPBASE, so that we avoid polluting the user's
name space when e.g. <sys/ptrace.h> is included. Previously, the
PC symbol in mips/regnum.h would conflict with the declaration of
the external variable by the same name in termcap.h, as discovered
by the ``okheaders'' regression test.
and use it preferentially to a value in pt_uc, clearing it once on the new
stack. Move stores into pt_uc back to before the stack switch; storing
after the stack switch opened a one-instruction race condition where an upcall
that had just started a chain could be preempted again, and would bomb when
restarted due to its pt_uc not yet having been updated. Now that pt_trapuc
is what the upcall code writes to, it is safe to store to pt_uc before
switching stacks.
Remove obsolete pt_sleepuc code.
perry