for gcc >= 2.5 and no-ops for gcc >= 2.6. Converted to use __dead2
or __pure2 where it wasn't already done, except in math.h where use
of __pure was mostly wrong.
Reduce _JBLEN for the i386 to what is actually used.
Encapsulate jmp_buf and sigjmp_buf in structs.
Enlarge jmp_buf to the same size as sigjmp_buf.
Declare *longjmp as non-returning.
Remove stale comments about sig*jmp not being implemented.
