Cache locations were documented inconsistently in three places.
The backup docs mentioned PATH being used to find fusermount, which is
never run by restic backup. It now mentions ssh and rclone, which are
used by backends.
The notion of a "system-wide" environment variable makes no sense.
TMPDIR is now mentioned because it allows for optimization and may
have security implications.
This commits adds rudimentary support for a cache directory, enabled by
default. The cache directory is created if it does not exist. The cache
is used if there's anything in it, newly created snapshot and index
files are written to the cache automatically.
