Fix behaviour of "udisks" variable.
If udisks is set to "auto", determine "udisks" as '1' or '0'.
If udisks is set to "1", exit with error if udisks is not available.
Without this change, "auto" and "1" are the same.
Shorten tests.
Simplify handling of read-only mounts.
Renamed variable "mount_options" to "default_mount_options".
The variable "mount_options" is modified temporarily for read-only mounts and
reset to "default_mount_options" afterwards.
Uncomment variables in bashmount.conf that are set to defaults anyway.