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/postProcessing/lumped/lumpedPointMovement/lumpedPointMovement.C

@@ -119,9 +119,8 @@ int main(int argc, char *argv[])
         "len",
         "Visualization length for planes (visualized as triangles)"
     );
-    argList::addBoolOption
+    argList::addDryRunOption
     (
-        "dry-run",
         "Test movement without a mesh"
     );
     argList::addBoolOption
@@ -142,7 +141,7 @@ int main(int argc, char *argv[])
     const label span   = Foam::max(1, args.getOrDefault<label>("span", 1));
 
     // Control parameters
-    const bool dryrun = args.found("dry-run");
+    const bool dryrun = args.dryRun();
     const bool slave = args.found("slave");
     const bool removeLock = args.found("removeLock");