Merge r1359884:

    make varbuf functions treat AP_VARBUF_UNKNOWN consistently, improve docs

    ap_varbuf_pdup(): copying the whole buffer in case strlen ==
    AP_VARBUF_UNKNOWN does not make sense as the caller can not set
    the exact buffer size, only a minimum. No API change as previously
    the behavior with AP_VARBUF_UNKNOWN was undocumented.

    regsub_core(): Checking for vb->buf is useless, it cannot be NULL
    unless ap_varbuf_init has not been called.

    ap_varbuf_cfg_getline(): Initially, allocate enough memory to hold
    an empty line. If strlen == AP_VARBUF_UNKNOWN, use strlen(buf) instead
    of undefined behavior.

Reviewed by: jim, sf, minfrin



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1418935 13f79535-47bb-0310-9956-ffa450edef68