command-line: Add function to print all options.

command-line: Add function to print all options.

This commit adds a function that prints (both long and short)
options of a ovs-* command.  To use this function, option
'--option' is added to ovs-appctl/dpctl/ofctl and ovsdb-tool
commands.  A future patch will use the option output to
conduct bash command-line completion.

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