ab: add the possibility to define a socket timeout (-s timeout)

[Guido Serra <zeph fsfe org>]

Also some changes In usage():
- reword -t option to be more clear with the new -s
- add missing -q
- add the new -s option
- reword some options to better match online documentation

ab.xml claimed that -s was already there, but nothing in the code looks like that.
So, I guess it only landed on the online doc...

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1422937 13f79535-47bb-0310-9956-ffa450edef68