Add poke commands

[mlafeldt]

Provided that the PS2 side supports it, add poke commands to ntpbclient to set the contents of memory at a specified address.

In particular, implement commands to poke byte, halfword, word, doubleword, quadword, and float:

• pokeb
• pokeh
• pokew (same as poke)
• poked
• pokeq
• pokef