Add %NOCANCEL flag to enable password entry for fuse mounts.

Hi there,

I was having problems where I couldn't enter a password for sshfs fuse mounts. This is apparently due
to vifm going into `noraw()` mode when `cancellable` is TRUE in `background_and_wait_for_status()`.

This patch adds a new flag for fuse definitions, `%NOCANCEL`, which effectively sets `cancellable` to false.

Cheers,
Johannes