services: monitoring: Add 'zabbix-server'.

services: monitoring: Add 'zabbix-server'.

* gnu/services/monitoring.scm (uglify-field-name, serialize-field,
serialize-number, serialize-list, serialize-string, group?, serialize-group,
include-files?, serialize-include-files, zabbix-server-account,
zabbix-server-config-file, zabbix-server-activation,
zabbix-server-shepherd-service, generate-zabbix-server-documentation,
extra-options, serialize-extra-options): New procedures.
(zabbix-server-service-type): New variable.
* gnu/tests/monitoring.scm (%psql-user-create-zabbix,
%psql-db-zabbix-create-script, %psql-db-create-zabbix, %psql-db-import-zabbix,
%zabbix-os, %test-zabbix): New variables.
(run-zabbix-server-test): New procedure.
* doc/guix.texi (Monitoring Services): Document 'zabbix-server'.

squash! services: monitoring: Add 'zabbix-server'.