Merge r1612945, r1734835, r1738875, r1833416, r1842898, r1867257, r1876745, r1876807...

Merge r1612945, r1734835, r1738875, r1833416, r1842898, r1867257, r1876745, r1876807, r1876823, r1877642 from trunk

Easy patches: synch 2.4.x and trunk
        - core: Re-introduce check for sufficient PCRE version.
        - core: Fix doc string for QualifyRedirectURL
        - mod_proxy: really return an error message on invalid "flusher" value.
        - mod_http2: Remove extra and un-needed ""
        - mod_ldap: fix a (unlikely) memory leak
        - ab: fix a typo
        - suexec: Report error string after failure from setgid/initgroups or setuid
        - mod_session_crypto: be less specific and don't echo passphrase
        - mod_proxy_html: Fix proxy_html_conf.bufsz to have correct type, as
                          it is used with ap_set_int_slot.
        - mod_md: update duplicated APLOGNOs.

Submitted by: fuankg, trawick, ylavic, jailletc36, jorton, jailletc36, jorton, covener, jorton, ylavic
Reviewed by: jailletc36, jim, ylavic
Backported by: jailletc36

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1879490 13f79535-47bb-0310-9956-ffa450edef68