This will allow us to use vshError() to report errors from inside
vshCommandOpt*(), instead of replicating the same logic and error
messages all over the place.

We also have more context inside the vshCommandOpt*() functions,
for example the actual value used on the command line, which means
we can produce more detailed error messages.

vshCommandOptBool() is the exception here, because it's explicitly
designed not to report any error.