src/utils_rrdcreate.c: Free "args".

Thanks to Yves Mettier for pointing this out!

Fixes: #661
python plugin: Don't create empty meta_data_t objects.

Fixes: #716
src/utils_format_json.c: Handle empty meta_data_t gracefully.

Previously, `meta != NULL` and `keys_num == 0` was possible and would
result in "{}", which is not valid JSON.

Fixes: 716
src/meta_data.c: If the meta_data_t is empty, avoid calling malloc(0).
Merge remote-tracking branch 'github/collectd-5.3' into collectd-5.3
curl plugin: Use the utility functions for config handling.
src/utils_match.[ch]: Implement match_value_reset().

This is called from the curl and memcachec plugins. Previously, gauge
average, minimum and maximum values were calculated since the start of
the daemon, rather than per-interval. This is not the documented
functionality.

Fixes: #663
network plugin: Improve client connecting behavior.

This moves the socket creation logic so it's called from
networt_send_buffer_plain(). This allows us to recover after network
failures or when collectd was started before the network was available.

Fixes: #627
Merge branch 'collectd-5.3' into collectd-5.4

Conflicts:
	src/curl.c