STYLE: foamPwd (posix) function with some env replacements (issue #844)

- the current working path with replacements for base-level
  OpenFOAM env variables such as FOAM_RUN, WM_PROJECT_DIR,
  WM_PROJECT_USER_DIR etc

  Can be used directly from the command-line or embedded into a
  command prompt. For example,

      PS1='$(foamPwd)\n\u\$ '

- aliases for user solver/utilities located under "$WM_PROJECT_USER_DIR":
  (ufoam, uapp, usol, uutil)