apr_uri_parse(): Do not accept invalid characters in the scheme.
Per RFC 3986 3.3, enforce that the first segment of a relative path does
not contain a colon.

PR: 52479


git-svn-id: https://svn.apache.org/repos/asf/apr/apr/trunk@1462224 13f79535-47bb-0310-9956-ffa450edef68