vty: Add ctrl-v <literal> to allow, e.g., '?' to be input for regex

* Support the standard ctrl-v <literal> control sequence. Otherwise there
  is no way to do this from the venerable telnet vty. vtysh supports this
  (configurably) via readline.
* lib/vty.c: (VTY_ESC_LITERAL) New mode, for ctrl-v.
  (vty_read) Additional mode to go ctrl-v -> VTY_ESC_LITERAL, and
  always insert next char.

Acked-by: Vincent.Jardin@6wind.com
diff --git a/doc/basic.texi b/doc/basic.texi
index 4485665..92e8d9e 100644
--- a/doc/basic.texi
+++ b/doc/basic.texi
@@ -552,6 +552,12 @@
 @kindex C-t
 Transpose character.
 
+@item C-v
+@kindex C-v
+Interpret following character literally. Do not treat it specially.
+This can be used to, e.g., type in a literal @kbd{?} rather than do
+help completion.
+
 @end table
 
 @node CLI Advanced Commands
@@ -593,4 +599,7 @@
 the line.  Typing @kbd{?} at any point in the line will show possible
 completions.
 
+To enter an actual @kbd{?} character rather show completions, e.g. to
+enter into a regexp, use @kbd{@key{C}-v ?}.
+
 @end table