ab: add SNI support when available.

ab: follow up to r1750854: put the -I at the right place for apr_getopt().

ab: follow up to r1750854.
Use SNI when available by default, and invert -I logic to now disable it.

ab: follow up to r1750854: some comments and better naming.

ab: follow up to r1750854: still better naming, and a C89 fix.
Submitted by: ylavic
Reviewed/backported by: jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1757674 13f79535-47bb-0310-9956-ffa450edef68