Make reloading extfs contents possible

[gotar]

Currently mc caches entire vfs structure and so requires restart to reload it (not only CmdReload doesn't reread original source, but also assumes some file operations as done even if they weren't).
This is both irritating and makes it impossible or hard to write some new extfses. I'm trying to use Android Debug Bridge to connect to my phone (without FISH or some fancy FUSE mounting like sshfs). However mc expects full directory listing (including all the subdirectories) at the very beginning, which is pointless in case of tree based layouts behind (not to mention it would take ages and put unnecessary strain to the device).
I'd like to use vfs reload code to fetch contents of cwd (or any currently required location only), just like it happens with regular fs (every cd gets fresh data). In order to do this I need two additional features:

1. some extfs marker to set for partial reload capable (for autoreloading contents after every cd, otherwise one needs to press ctrl-r),
2. passing internal cwd (i.e. the part after #extfs) to the list action.

[slavazanko]

Closely related to #62

[slavazanko]

(In #2695) Merged to master:

git log --pretty=oneline 963ee34...4047403