backend determination incomplete, not configurable

As of byobu 4.32 (I think it was introduced in 4.30), there is an "/etc/byobu/backend" config file to choose between screen and tmux. However, it does not seem to be read at all, neither is "$BYOBU_CONFIG/backend" (see the output of "bash -x /usr/bin/byobu" [1])

It looks like you prefer using symlinks to /usr/bin/byobu instead ("byobu-tmux" and "byobu-screen"), but:
1/ these don't get installed by "make install" (and sorry, I don't feel like investigate the autotools...)
2/ I like to be able to set the backend in a config file too, to be able to use "byobu" as usual.

The attached patch "byobu_fix-tmux.patch" fixes this second point. The backend can thus be set by the following methods, from more prioritized to less prioritized:
* using "byobu-tmux" or "byobu-screen" instead of just "byobu";
* setting the BYOBU_BACKEND environment before launching "byobu";
* setting BYOBU_BACKEND in "$BYOBU_CONFIG/backend";
* setting BYOBU_BACKEND in "/etc/byobu/backend";
* falling back to screen if none of the previous methods were applicable.

which means that if one of these methods is used, all methods below are not taken into account.

The second patch "byobu_doc.patch" is an update to various places of documentation ("/etc/byobu/backend", "/usr/share/byobu/status/status" and the "byobu.1" man page).
Note about doc I *haven't* updated: the "byobu.1" man page is fairly out-of-date, since it only is related to screen. Also, you might want to integrate the various methods available to set the backend, along with their precedence.

Thank you!

[1] http://pastebin.archlinux.fr/433760