vty: enable optional-multi-choice syntax: ([one]|[two])

Add basic optional multi-choice argument support.

The VTY detects optional arguments by square braces.

 > cmd ?
 [optional-arg]
 > cmd optional-arg
 ok
 > cmd
 ok

However, within multi-choice args, these braces were so far not treated as
optional:

 > list
 cmd2 ([one]|[two]|[three])
 > cmd2
 % Command incomplete

In preparation for I952b3c00f97e2447f2308b0ec6f5f1714692b5b2 which will enable
the more obvious syntax of

  cmd [(one|two)]

for reasons of internal implementation, first support a syntax of

  cmd ([one]|[two])

The internal vty implementation always needs square braces around each option.
There is currently no good way to prevent developers from defining braces
inside multi-arguments, so it is easiest to allow and handle them:

 > list
 cmd2 ([one]|[two]|[three])
 > cmd2
 ok

The VTY doesn't guard against a mix like

 cmd (one|[two])

With this patch, a multi-choice command is treated as optional iff the first
element is in square brackets. The remaining elements' square brackets have no
effect besides confusing the user. This is not explicitly checked against.

In general, I would prefer to check all of these details, but the current VTY
code with its endless code duplication and obscure string mangling just doesn't
provide that luxury. There are numerous worse errors hidden in there.

Change-Id: I9a8474bd89ddc2155c58bfca7bd038d586aaa60a

0 files changed, 0 insertions, 0 deletions