command-line: Add function to print command usage.

command-line: Add function to print command usage.

This commit adds a new variable in 'struct command' for
recording the command usage.  Also, a new function is
added to print the usage given the array of defined
commands.

Later patch will use the output in bash command-line
completion script.

Signed-off-by: Alex Wang <alexw@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>