* gnu/services/networking.scm (ntp-server->string): Use the textual
representation of the values as printed by 'display' rather than 'write', to
avoid inserting double quotes in the generated config.
* tests/networking.scm (%ntp-server-sample): Add a comment and make one of the
options a string, to exercise the fix.
("ntp-server->string"): Move the expected value to the first argument.
("ntp configuration servers deprecated form"): Likewise.
("openntpd generated config string ends with a newline"): Likewise.

See http://debbugs.gnu.org/cgi/bugreport.cgi?bug=37318.

* gnu/services/networking.scm (openntpd-configuration->string): New procedure,
extracted from top of the `openntpd-shepherd-service' to make it testable.
(openntpd-shepherd-service): Adapt following the move of the code to the above
procedure.
* tests/networking.scm: Add a test for the `openntpd-configuration->string'
procedure.

* gnu/services/networking.scm (ntp-server-types): New enum.
(<ntp-server>): New record type.
(ntp-server->string): New procedure.
(%ntp-servers): Define in terms of <htp-server> records.  Use the first
entrypoint server as a pool instead of a list of static servers.  This is more
resilient since a new server of the pool can be interrogated on every
request.  Add the 'iburst' options.
(ntp-configuration-servers): Define a custom accessor that warns but honors
the now deprecated server format.
(<ntp-configuration>): Use it.
(%openntpd-servers): New variable,
(<openntpd-configuration>): Use it, as a pool ('servers' field) instead of a
regular server.
* tests/networking.scm: New file.
* Makefile.am (SCM_TESTS): Register it.
* doc/guix.texi: Update documentation.