v2.19: C Migrations and Bug Fixes

Bug Fixes:

* Fix remote timeout in delta restore. When performing a delta restore on a largely unchanged cluster the remote could timeout if no files were fetched from the repository within protocol-timeout. Add keep-alives to prevent remote timeout. (Reported by James Sewell, Jens Wilke.)
* Fix handling of repeated HTTP headers. When HTTP headers are repeated they should be considered equivalent to a single comma-separated header rather than generating an error, which was the prior behavior. (Reported by donicrosby.)

Improvements:

* JSON output from the info command is no longer pretty-printed. Monitoring systems can more easily ingest the JSON without linefeeds. External tools such as jq can be used to pretty-print if desired. (Contributed by Cynthia Shang.)
* The check command is implemented entirely in C. (Contributed by Cynthia Shang.)

Documentation Improvements:

* Document how to contribute to pgBackRest. (Contributed by Cynthia Shang.)
* Document maximum version for auto-stop option. (Contributed by Brad Nicholson.)

Test Suite Improvements:

* Fix container test path being used when --vm=none. (Suggested by Stephen Frost.)
* Fix mismatched timezone in expect test. (Suggested by Stephen Frost.)
* Don't autogenerate embedded libc code by default. (Suggested by Stephen Frost.)

test/lib/pgBackRestTest/Common/CodeCountTest.pm

@@ -168,7 +168,7 @@ sub codeCountScan
         my $strYaml = executeTest(
             "cloc --yaml ${strBasePath}/${strFile}" .
             " --read-lang-def=${strBasePath}/test/code-count/code.def" .
-            " --force-lang='${strForceLang}'");
+            " --force-lang='${strForceLang}'", {bSuppressStdErr => true});
 
         # Error if the file was ignored
         if ($strYaml =~ '1 file ignored')