Use db_printsym() to display function names in stack traces.

Previously, the stack unwinder tried to locate the start of the function
in each frame by walking backwards until it found an instruction that
modified the stack pointer and then assumed that was the first instruction
in a function. The unwinder would only print a function name if the
starting instruction's address was an exact match for a symbol name.
However, not all functions generated by modern compilers start off functions
with that instruction. For those functions, the unwinder would fail to
find a matching function name. As a result, most frames in a stack
trace would be printed as raw hex PC's instead of a function name.

Stop depending on this incorrect assumption and just use db_printsym()
like other platforms to display the function name and offset for each
frame. This generates a far more useful stack trace.

While here, don't print out curproc's pid at the end of the trace. The
pid was always from curproc even if tracing some other process.

In addition, remove some rotted comments about hardcoded constants that
are no longer hardcoded.

Sponsored by: DARPA / AFRL