This allows mapped devices listed in 'dependencies' to be properly taken
into account.
Reported by Andreas Enge <andreas@enge.fr>.
* gnu/system.scm (mapped-device-user): Check whether DEVICE is a member
of the 'dependencies' of FS.
* tests/system.scm (%luks-device, %os-with-mapped-device): New variables.
("operating-system-user-mapped-devices")
("operating-system-boot-mapped-devices")
("operating-system-boot-mapped-devices, implicit dependency"): New tests.
