Backport r1480067 to v1.5 branch.
Add the apr_table_getm() call, which transparently handles the
merging of keys with multiple values.


git-svn-id: https://svn.apache.org/repos/asf/apr/apr/branches/1.5.x@1480071 13f79535-47bb-0310-9956-ffa450edef68