This changes the 'custom-serializer' field into a generic
'extra-args' field that can be extended to support new literals.
Within extra-args, the literals 'sanitizer' and 'serializer' allow
for user-defined sanitization and serialization procedures respectively.
The 'empty-serializer' was also added as a literal to be used as before.
To prevent confusion between the new “explicit” style of specifying
a sanitizer, and the old “implicit” style, the latter has been
deprecated, and a warning is issued if it is encountered.
* gnu/services/configuration.scm (define-configuration-helper):
Rename 'custom-serializer' to 'extra-args'. Add support for literals
'sanitizer', 'serializer' and 'empty-serializer'. Rename procedure
'field-sanitizer' to 'default-field-sanitizer' to avoid syntax clash.
Only define default field sanitizers if user-defined ones are absent.
(normalize-extra-args): New variable.
(<configuration-field>)[sanitizer]: New field.
* doc/guix.texi (Complex Configurations): Document the newly added
literals.
* tests/services/configuration.scm: Add tests for the new literals.
Signed-off-by: Liliana Marie Prikler <liliana.prikler@gmail.com>