Add missing Server-Variables useable in RewriteCond directive.
Introduced in r1132494
   CONTEXT_PREFIX
   CONTEXT_DOCUMENT_ROOT
Introduced in r737973
   IPV6
Missing for ages!
   SCRIPT_GROUP
   SCRIPT_USER

I have added where I found it logical, feel free to adjust.
I have also reordered this table to ease reading.
Finally, I have beautified some tables at the end.

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Remove useless link.

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- Fix extra ] spotted in online doc by emigas.
- Remove another ] introduced in Hearder syntax in r1588244
- s/variable/varname/ to match the text below it.
- Turn the syntax of both Header and RequestHeader to be:
   header [value [replacement [early|env=[!]variable|expr=expression]]] 
in order to match the code.
This removes the ambiguity of expr=<something> which can now be used in 2 different places in the syntax.
- Also remove the space in 'expr= value' in compatibility note, to help distinguish between the two

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mod_proxy_scgi: Support Unix sockets.

ap_proxy_port_of_scheme(): Support default SCGI port (4000).

Submitted by: trawick
Reviewed/backported by: jim


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        programs, and implement a special merging algorithm for
             SSLCertificate[Key]File to emulate the behavior in versions <= 2.4.7



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ssl_stapling_init_cert: do not return success when no responder URI is found
stapling_renew_response: abort early (before apr_uri_parse) if ocspuri is empty

Submitted by: kbrand
Reviewed/backported by: jim


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ssl_callback_TmpDH: for OpenSSL 1.0.2 and later, set the current cert to the
one actually used for the connection before calling SSL_get_privatekey(ssl)

Submitted by: kbrand
Reviewed/backported by: jim


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Fix errors with CacheLock on Windows:

cache_util.c(757): (OS 80)The file exists.  : [client 127.0.0.1:63889] 
AH00784: Attempt to obtain a cache lock for stale cached URL failed, 
revalidating entry anyway: 

Submitted by: covener
Reviewed/backported by: jim


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mod_expires: don't add Expires header to error responses (4xx/5xx),
             be they generated or forwarded. PR 55669.


Follow up to r1584430.

Submitted by: ylavic
Reviewed/backported by: jim


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mod_proxy_fcgi: Don't segfault when failing to connect to the backend.


now understood why users haven't reported the segfault (yet) when mod_proxy_fcgi
can't connect to the application

Submitted by: trawick
Reviewed/backported by: jim


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                   on startup if default socache_provider is used and
                   AuthnCacheEnable or AuthnCacheProvideFor is used.
                   This problem has been introduced in r1531961.
                   PR 56371.

Submitted by: jkaluza
Reviewed by: ylavic, jim, trawick


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$ ./test/readbody.sh httpd scgi tcp 10000 100 2>&1 | egrep '(Document Length|Failed requests|Requests per second)'
Document Length:        71453 bytes
Failed requests:        0
Requests per second:    9698.92 [#/sec] (mean)
$ ./test/readbody.sh httpd scgi unix 10000 100 2>&1 | egrep '(Document Length|Failed requests|Requests per second)'
Document Length:        71453 bytes
Failed requests:        0
Requests per second:    10140.24 [#/sec] (mean)
$ ./test/readbody.sh nginx scgi tcp 10000 100 2>&1 | egrep '(Document Length|Failed requests|Requests per second)'
Document Length:        71453 bytes
Failed requests:        0
Requests per second:    3900.32 [#/sec] (mean)
$ ./test/readbody.sh nginx scgi unix 10000 100 2>&1 | egrep '(Document Length|Failed requests|Requests per second)'
Document Length:        71453 bytes
Failed requests:        0
Requests per second:    4300.62 [#/sec] (mean)


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