Add mg_http_send_redirect and make examples use it

docs/c-api/http_server.h/intro.md

docs/c-api/http_server.h/mg_http_send_redirect.md

docs/c-api/http_server.h/mg_send_response_line.md

docs/c-api/util.h/intro.md

docs/c-api/util.h/mg_asprintf.md

docs/c-api/util.h/mg_avprintf.md

examples/connected_device_2/server.c

examples/connected_device_3/server.c

examples/connected_device_4/server.c

examples/cookie_auth/cookie_auth.c

mongoose.c

mongoose.h