ENH: argList improvements

- argList::envExecutable() static method.
  This is identical to getEnv("FOAM_EXECUTABLE"), where the name of
  the executable has typically been set from the argList construction.

  Provides a singleton access to this value from locations that
  do not have knowledge of the originating command args (argList).
  This is a similar rationale as for the argList::envGlobalPath() static.

- additional argList::envRelativePath() static method.

- make -dry-run handling more central and easier to use by adding into
  argList itself.

STYLE: drop handling of -srcDoc (v1706 option)

- replaced with -doc-source for 1712 and never used much anyhow

applications/utilities/preProcessing/foamUpgradeCyclics/foamUpgradeCyclics.C

@@ -404,9 +404,8 @@ int main(int argc, char *argv[])
     timeSelector::addOptions();
 
     argList::addOptionCompat("dry-run", {"test", 1806});
-    argList::addBoolOption
+    argList::addDryRunOption
     (
-        "dry-run",
         "Test only do not change any files"
     );
     argList::addBoolOption