top(1): move command mapping to commands.c

top(1): move command mapping to commands.c

This eliminates the difficult to follow mapping of a string list. It
moves numbers from "#define" into (more) debuggable enums. More
generally, it follows the trend of moving more data into a more central
mechanism.

The help output is a little worse: " " is not rendered well, and there
are duplicate entries, but that will be fixed in a followup.