help: use man viewer path from "man.<tool>.path" config var

help: use man viewer path from "man.<tool>.path" config var

This patch implements reading values from "man.<tool>.path"
configuration variables, and using these values as pathes to
the man viewer <tool>s when lauching them.

This makes it possible to use different version of the tools
than the one on the current PATH, or maybe a custom script.

In this patch we also try to launch "konqueror" using
"kfmclient" even if a path to a konqueror binary is given
in "man.konqueror.path".

The "man_viewer_list" becomes a simple string list to simplify
things for the following patches.

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