CONFIG: eliminate most occurances of outdated FOAM_INST_DIR (issue #444)

- since 1612, FOAM_INST_DIR and foamInstDir longer have any
  special meanings when sourcing the bashrc or cshrc files.
  Thus no need for special treatment in any of the dispatch wrappers.

  Retained FOAM_INST_DIR as (unexported) variable in etc/bashrc,
  just in case people are using patched versions of etc/bashrc
  as part of their installation.

ENH: relax prefix restrictions on foamCreateVideo (issue #904)

- shift the implicit '.' to be part of the default prefix. This allows
  things like "-image myimages_00" to work as might be expected.