ENH: add wmake -user option

- overrides values of FOAM_APPBIN and FOAM_LIBBIN with those from
  FOAM_USER_APPBIN and FOAM_USER_LIBBIN, respectively.

  It can be useful in some situations to compile code into user locations
  rather than library locations. In #1522, for example, it will
  normally be useful to deliver out packages that compile into
  FOAM_LIBBIN (when included as a module) but still allow the user the
  chance to install into their own directories without editing all of
  the Make/files.