Support for accessing arbitrary MIPS stack frames in memory.

Support for accessing arbitrary MIPS stack frames in memory.

* blockframe.c (get_prev_frame_info):  If INIT_FRAME_PC_FIRST is
set, run it before INIT_EXTRA_FRAME_INFO.
* stack.c (frame_info):  If PRINT_EXTRA_FRAME_INFO defined, call it.

* mips-tdep.c (init_extra_frame_info): Only clobber the `frame'
(FP) value in the frame_info struct if it is zero (as from top of
execution stack).
(setup_arbitrary_frame):  Implement FRAME_SPECIFICATION_DYADIC.

* mips-xdep.c (fetch_inferior_registers):  ZERO_REGNUM always
comes back as zero.  So does FP_REGNUM, as a trigger for
init_extra_frame_info.

* tm-mips.h (INIT_FRAME_PC_FIRST):  Kludge, FIXME, defined to get
the program counter set before INIT_EXTRA_FRAME_INFO is run.
(INIT_FRAME_PC):  Defined to null.
(PRINT_EXTRA_FRAME_INFO): print frame pointer location via symtab.
(FRAME_SPECIFICATION_DYADIC):  Ask for two args in frame command.
Briefly explain MIPS stacks in GDB.