ENH: various general improvments to the POSIX functions

- ensure proper and sensible handling of empty names.
  Eg, isDir(""), isFile("") are no-ops, and avoid file-stat

- rmDir:
  * optional 'silent' option to suppress messages.
  * removes all possible sub-entries, instead of just giving up on
    the first problem encountered.

- reduced code duplication in etcFiles

ENH: provide WM_USER_RESOURCE_DIRNAME define (in foamVersion.H)

- this is still a hard-coded value, but at least centrally available