api: Add API call for external bind mounts

This one is to bind-mount host directory into container on start.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
fs: Add flags argument to add_mount call

In the future we'll add at least RO one for sure.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
ct: Move net and cg structures into proper headers

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
fs: Move mount/umount code into fs.c helpers

We'll add bin dmounts and MOUNTED state, so this is
handy to have this code separated.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
fs: Add fs_mount object and its manipulations

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
fs: Fix misorder in bind mount arguments

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
fs: Initial implementation of external bind mounts

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>