linux-initrd: Return file-like objects instead of monadic values.

linux-initrd: Return file-like objects instead of monadic values.

This is an incompatible change visible to users via the 'initrd' field
of 'operating-system'.  However, assuming the user's 'initrd' value
tail-calls to 'raw-initrd' or 'base-initrd', the switch to non-monadic
style is invisible.

* gnu/system/linux-initrd.scm (expression->initrd): Use 'computed-file'
instead of 'gexp->derivation'.
(raw-initrd, base-initrd): Adjust docstring to mention non-monadic
return.
* gnu/system/vm.scm (expression->derivation-in-linux-vm): Adjust
accordingly.
* gnu/system.scm (operating-system-directory-base-entries)
(operating-system-initrd-file)
(operating-system-boot-parameters): Adjust accordingly.
* doc/guix.texi (operating-system Reference)
(Initial RAM Disk): Update.