* gnu/system/privilege.scm (<privileged-program>): Add a field
representing the program's POSIX capabilities.
(privileged-program-capabilities): New public procedure.
* doc/guix.texi (Privileged Programs): Document it.
* gnu/build/activation.scm (activate-privileged-programs): Take a LIBCAP
package argument providing setcap(8) to apply said capabilities.
* gnu/services.scm (privileged-program->activation-gexp): Pass said
package argument where supported. Include privileged-program-capabilities
in the compatibility hack.