vtysh: Extending vtysh to allow question-mark cmds

vtysh: Extending vtysh to allow question-mark cmds

Currently, "vtysh -c" interface does not provide a logic to parse
commands ending with '?' character. In consequence, the following behavior
is observed:

$ vtysh -c "show bgp ?"
 % Unknown command.

With these changes, i'm extending FRR's parser to be able to handle
these commands, which allow a more friendly interaction with users
that rely on "vtysh -c" interface. The typical use-case here is for
scenarios in which the final users relie on external/their-own CLI and
require a friendly interface to FRR's vtysh cli.

$ vtysh -c "show bgp ?"
  <cr>
   A.B.C.D               Network in the BGP routing table to
   display
   A.B.C.D/M             IPv4 prefix
   X:X::X:X              Network in the BGP routing table to display
   X:X::X:X/M            IPv6 prefix
   attribute-info        List all bgp attribute information
   cidr-only             Display only routes with non-natural netmasks
   community             Display routes matching the communities
   community-info        List all bgp community information
 ...

Signed-off-by: Rodny Molina <rmolina@linkedin.com>