configure: Allow GIT_ARG_SET_PATH to handle --without-PROGRAM

configure: Allow GIT_ARG_SET_PATH to handle --without-PROGRAM

Add an optional second argument to both GIT_ARG_SET_PATH and
GIT_CONF_APPEND_PATH such that any value of the second argument will
enable configure to set NO_$PROGRAM in addition to an empty
$PROGRAM_PATH.  This is initially useful for allowing configure to
disable the use of python, as the remote helper code has nothing
leveraging it yet.

The Makefile already recognizes NO_PYTHON, but configure provided no
way to set it appropriately.

Signed-off-by: Ben Walton <bwalton@artsci.utoronto.ca>
Signed-off-by: Junio C Hamano <gitster@pobox.com>