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/miscellaneous/foamRestoreFields/foamRestoreFields.C

@@ -5,7 +5,7 @@
     \\  /    A nd           | www.openfoam.com
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
-    Copyright (C) 2018-2020 OpenCFD Ltd.
+    Copyright (C) 2018-2021 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -228,9 +228,8 @@ int main(int argc, char *argv[])
         "In serial mode use times from processor0/ directory, but operate on "
         "processor\\d+ directories"
     );
-    argList::addBoolOption
+    argList::addDryRunOption
     (
-        "dry-run",
         "Report action without moving/renaming"
     );
     argList::addBoolOption
@@ -247,7 +246,7 @@ int main(int argc, char *argv[])
 
     #include "setRootCase.H"
 
-    dryrun = args.found("dry-run");
+    dryrun = args.dryRun();
     verbose = args.found("verbose");