* mipsread.c (fixup_sigtramp): Also look for _sigtramp as a real

* mipsread.c (fixup_sigtramp):  Also look for _sigtramp as a real
routine (for Irix-4.x).  Make many funcs static and void.
* mips-tdep.c (mips-frame-chain):  Clean up, simplify.
* (init_extra_frame_info):  Don't trash cached value of frame
pointer register.  This fixes backtracing through routines that use
alloca().  Generally clean up declarations of functions, and use
typedefs and macros to reference data structures as appropriate.
* tm-irix3.h, tm-mips.h (EXTRA_FRAME_INFO):  use proper type for
proc_desc element.