rgw: make init env methods return an error

rgw:  make init env methods return an error

Since web frontends may signal an error when requests are malformed or so, let
us double check this and raise errors early. The current user of this is
civetweb frontend; which can potentially return null from `parse_http_headers`
when a HTTP header without a ":" is supplied at which point headers.value is
null which can lead to undefined behaviour later in RGW.

Fixes: http://tracker.ceph.com/issues/23039
Signed-off-by: Abhishek Lekshmanan <abhishek@suse.com>
(cherry picked from commit 7872a831783e17dcc4d0aa70cffc256afb664f7a)

src/rgw/rgw_lib.h:
  Added https://github.com/ceph/ceph/pull/19065 to the backport as it is harmless