zabbix: reorganize files dir for clarity and to better follow the FHS
Reorganize the extra files under the 'files' dir so that it is more
clear where they go and what they are for, and to avoid having a
disorganized mass of files.
Move the extra C source file out of files and into 'src-extra'.
Avoid treating openwrt-supplied params files as configuration data.
Instead have them in a separate directory under /usr/share which is
included in the zabbix_agentd configuration by default. This also better
follows the FHS.
Hopefully these changes make sense on their own, as well as helping the
CI accept the changes.