ExecuteCommand was becoming pretty confusing with unused variables
for some commands, confusing names (device ID != IOS file descriptor),
duplicated checks, not keeping the indentation level low, and having
tons of things into a single function.
This commit gives more correct names to variables, deduplicates the
device checking code, and splits ExecuteCommand so that it's
easier to read.
It's worth noting that some device checks have been forgotten in the
past, which has caused a bug (which was recently fixed in 288e75f6).
b65ad538ba