help: implement multi-valued "man.viewer" config option

help: implement multi-valued "man.viewer" config option

This allows multiple viewer candidates to be listed in the configuration
file, like this:

        [man]
                viewer = woman
                viewer = konqueror
                viewer = man

The candidates are tried in the order listed in the configuration file,
and the first suitable one (e.g. konqueror cannot be used outside windowed
environment) is used.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Tested-by: Xavier Maillard <xma@gnu.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>