services: Move configuration functions that shouldn't be factorized.

* gnu/services/configuration.scm (serialize-field, serialize-string)
(serialize-space-separated-string-list, space-separated-string-list?)
(serialize-file-name, file-name?, serialize-boolean): Move these functions...
* gnu/services/cups.scm: ...to this file.
* gnu/services/kerberos.scm: ...to this file.

Configuration syntaxes are very specific to services.  Some services may have
the same configuration syntax, but none of them is common enough to be
abstracted in configuration.scm.

Signed-off-by: Clément Lassieur <clement@lassieur.org>