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>
ovs-appctl: Rename 'help' to 'list-commands'.

Having 'ovs-appctl help' and 'ovs-appctl --help' print different
output is confusing.  This commit renames the 'help' to 'list-commands'.
Also, future patches will add the 'list-commands' to other ovs-*
commands, and the output will be used by bash command-line completion
script.

Signed-off-by: Alex Wang <alexw@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>
dpctl: Implement 'list-commands' function.

This commit implements the 'list-commands' command for ovs-dpctl
and ovs-appctl dpctl/* commands.  The function will print the
usage string for each subcommand.

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