zabbix: add full variants and add-on support for other

zabbix: add full variants and add-on support for other

Rather than having a database selection for SQLITE which prevents
the server or frontend from building, we add a 'basic'
variant for the proxy which uses sqlite3, and have the database
Kconfig affect only the server and frontend.

* There are now only three variants:
  1. full, which is the default. It includes the full monitoring feature
     set currently available on openwrt, including netsnmp, curl-based
     checks, and ldap. In addition these features, plus the choice of
     database and ssl provider (or no ssl) are configuration options for
     this variant.
  2. basic, which provides basic functions with openssl support
  3. no-configure, for packages which are not part of the main Zabbix
     compile process (including the WebUI which only requires copying
     files for use by a web server with PHP CGI support).

* Full is the default variant for agentd and proxy, which are the only
  packages with a choice between full and basic. All other packages only
  are part of one variant.

* Full variants are the base version of the packages (that is
  zabbix-agentd is the 'full' version while zabbix-agentd-basic is the
  core version). The proxy version is named zabbix-proxy-basic-sqlite to
  announce that it is using the sqlite3 database and not a database
  server.

* get and sender only build if at least one of agentd, server, or proxy
are built. Therefore prevent selection get or sender when they would not
build.

* Zabbix's use of NetSNMP requires that Zabbix be build with OpenSSL

* While we are here, enable support for dates after 2038 (64-bit time_t)

* https://github.com/openwrt/packages/pull/28585#issuecomment-3984978895
  * we updated the name to reflect that it is for basic functionality
    that can standalone, rather then being a core the other packages
    build on.
  * basic has been used rather than tiny or small since the sentence
    'Provides only tiny/small functionality with SSL/TLS' in the
    description, sounds strange, but using basic this reads properly.

Signed-off-by: Daniel F. Dickinson <redacted>