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