Coverity points out that we aren't checking the return value
from curl_easy_setopt() for any of the calls to it we make
in block/curl.c.
Tested with 'make check' and with some basic smoke test command lines
suggested by Dan:
qemu-img info
https://cloud.debian.org/images/cloud/buster/daily/latest/debian-10-nocloud-amd64-daily.qcow2
qemu-img info --image-opts
driver=qcow2,file.driver=https,file.url=https://cloud.debian.org/images/cloud/buster/daily/latest/debian-10-nocloud-amd64-daily.qcow2
Changes v1->v2:
* new patch 1 which fixes a missing "set the error string" for
when curl_init_state() returns failure, since we're about to
add more cases when that function can fail
* set the error string in the failure path for the direct setopt
calls in curl_open()
* fix the failure path in curl_setup_preadv() by putting
the curl_easy_setopt() call in the same if() condition
as the existing curl_multi_add_handle()