vtysh: Handle `en` better when in -u for vtysh

vtysh: Handle `en` better when in -u for vtysh

vtysh was unable to distinguish between end and ena.  The
code can now do so:

sharpd@eva ~/frr5 (master)> sudo vtysh/vtysh -u sharpd

Hello, this is FRRouting (version 8.1-dev).
Copyright 1996-2005 Kunihiro Ishiguro, et al.

eva> e
% Ambiguous command: e
eva> en
% Command not allowed: enable
eva> ena
% Command not allowed: enable
eva> enab
% Command not allowed: enable
eva> enabl
% Command not allowed: enable
eva> enable
% Command not allowed: enable
eva> enb
% Unknown command: enb
eva> enc
% Unknown command: enc
eva> end
% Unknown command: end
eva> ene
% Unknown command: ene
eva> quit

Fixes: #2296
Signed-off-by: Donald Sharp <sharpd@nvidia.com>