Add MG_HTTP_INDEX build var

Update ca.pem links

Increase sntp test timeout

Add mg_mqtt_next_unsub() signature

Handle calloc failures

Rename, use hyphen

Merge branch 'dev'

mongoose.h 6.17

Merge pull request cesanta#1020 from nliviu/patch-2

Fix ASAN build for examples/cookie_auth

Merge branch 'dev'