Enhance the ddb examine (x) command.

• 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.

This is based on the similar commit from DragonFlyBSD without the
cosmetic changes.

Relnotes: yes
Obtained from: DragonflyBSD (Matthew Dillon)
Reference: 0624d20e86affcd708609cbf9014207537537a72