services: messaging: Prosody config supports file-like objects.

* doc/guix.texi (Messaging Services): Update accordingly.
* gnu/services/configuration.scm (serialize-configuration,
serialize-maybe-stem, serialize-package): Return strings or string-valued
gexps (these procedures were only used for their side-effects).
* gnu/services/messaging.scm (serialize-field, serialize-field-list,
enclose-quotes, serialize-raw-content, serialize-ssl-configuration,
serialize-virtualhost-configuration-list,
serialize-int-component-configuration-list,
serialize-ext-component-configuration-list,
serialize-virtualhost-configuration, serialize-int-component-configuration,
serialize-ext-component-configuration, serialize-prosody-configuration):
Return strings or string-valued gexps and stop printing.
(prosody-activation): Use SERIALIZE-PROSODY-CONFIGURATION's return value with
MIXED-TEXT-FILE instead of using its output with PLAIN-FILE.
(serialize-non-negative-integer, serialize-non-negative-integer-list): Convert
numbers to strings.
(file-object?, serialize-file-object, file-object-list?,
serialize-file-object-list): New procedures.
(ssl-configuration)[capath, cafile], (prosody-configuration)[plugin-paths,
groups-file]: Replace FILE-NAME with FILE-OBJECT.
* guix/gexp.scm (file-like?): New exported procedure.