* gnu/services/base.scm (swap-service-type): Do not include 'udev' requirement
for the Hurd. Use system* with "swapon", "swapoff" for the Hurd.
* gnu/system.scm (hurd-default-essential-services): Add swap-services.
* gnu/services/base.scm (swap-service-type):
Change-Id: I1d4d445c614921752dc84aa0dd6ff42cdbf62aa8