ENH: support default/alternate values for env-vars in dictionary lookup

- was previously only within string expansions, but cover dictionaries
  as well for consistency

ENH: replace the never-used fileName::caseName() functionality

- stringOps::inplaceReplaceVar() is more general

     stringOps::inplaceReplaceVar(myfile, "FOAM_CASE");

STYLE: relax parameter passing when calling some POSIX 'query' functions.

- A std::string is sufficient since the functions use a plain C-string.
  Eg, getEnv("SOMETHING").
  Retain more stringent Foam::word for things like setEnv, since this
  could be useful.