- fix overly aggressive match in the API value

- allow `INTELMPI*` generic value, this can be used to specify something
  like INTELMPI_custom and populate the corresponding wmake rule
  manually

STYLE: mention FOAM_BUILDROOT in wmake -help-full output

STYLE: adjust openfoam shell session welcome information

- adjust internal variable names to reduce collision potential

- improve handling of openfoam -etc=...