Restructure client/ subdir. Add two different flavors of client.
The full featured birdc client code is in client/birdc/.
The new light client birtcl is in client/birdcl/.
Common sources of both clients are directly in client/.
Rework on-line auto-completion in client/command.c to conditionally turn off
ncurses-specific code.
Add lightweight client without libreadline and ncurses dependencies - birdcl.
The birdcl lacks support of history, on-line auto-completion and there
are different implementations of "more" functionality and help on '?' press.
New client operates in canonical terminal mode (apart from "more" display)
and therefore all commands have to be executed by a return key including help
commands (called by '?' character in the end of the line).
Apart from these limitations the interaction style should be the same as
for the full client - birdc.
Build of birdcl is always on (independent on --enable-client parameter).
