vtysh: don't append superflous spaces (BZ#750)

vtysh: don't append superflous spaces (BZ#750)

rl_completion_append_character is reset to space every time the completion
function is entered. So we would have to set it to '\0' every time
new_completion() is called. We can make this conditional and avoid using
rl_pending_input.

This code path is most relevant when there are multiple completion
matches with the same prefix, e.g. in router bgp context: "neighbor 1.2.3.4
pa"<ssive|ssword> would have been completed to "neighbor 1.2.3.4 pass "
instead of "neighbor 1.2.3.4 pass".

Signed-off-by: Christian Franke <chris@opensourcerouting.org>
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>