Add V.34 option for ModemStandard directive

("i-was-rich-and-got-a-leased-line" is still undocumented, but do we really to document it?)

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1731349 13f79535-47bb-0310-9956-ffa450edef68