Enhance the ddb examine (x) command.

[pfg]

Summary

• Change x/a to work similar to gdb. The content of the memory is treated as an address, printed symbolically and the address is advanced. This way you can x/a <stack_address> and then just hit return a bunch of times to locate useful data on the stack.

• Add x/p. The content of the memory is treated as an address and printed as hex.

Obtained from: DragonflyBSD.
I obviated the cosmetic fixes from:
http://gitweb.dragonflybsd.org/dragonfly.git/blobdiff/df0cd41ca2b32c997e05f2ce18551386dda44055..0624d20e86affcd708609cbf9014207537537a72:/sys/ddb/db_examine.c

Test Plan

builds.

[pfg]

Copy/paste oops ...

[pfg]

Minor indentation change -- for consistency with existing style.

[pfg]

Committed as r298938.
This should let us advance in other changes.
I didn't mention this Differential Revision since there were no comments.

[pfg]

Actually, I ended up reverting this because this revision depends on this DragonFly change

kernel - Change where dump context is saved

• Save the dump context a little earlier in the panic to improve chances that post-morten kgdb can print the stack backtrace.
• Use a function union for variable arguments calls from ddb>

http://gitweb.dragonflybsd.org/dragonfly.git/commit/adf0eb4f774a65d9107038289bd52f21cb0d05d3