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/parallelProcessing/decomposePar/decomposePar.C

@@ -325,9 +325,8 @@ int main(int argc, char *argv[])
 
     #include "addAllRegionOptions.H"
 
-    argList::addBoolOption
+    argList::addDryRunOption
     (
-        "dry-run",
         "Test without writing the decomposition. "
         "Changes -cellDist to only write VTK output."
     );
@@ -410,7 +409,7 @@ int main(int argc, char *argv[])
 
     #include "setRootCase.H"
 
-    const bool dryrun           = args.found("dry-run");
+    const bool dryrun           = args.dryRun();
     const bool writeCellDist    = args.found("cellDist");
     const bool verbose          = args.found("verbose");