Implement Greg's mode recognition

[Andreas Schnellbacher]

Greg Jarvis changed the previous NEPMD mode recognition to add
the following:

o recognize Unix shebang
o better mode detection
o ...

Unfortunately the provided code has at least one severe bug,
that I'm not able to debug in shortness. That might be the
reason that Greg had problems to get it implemented for some
modes in the past. Moreover, it would lead to a much slower mode
detection as it was before. The method before was also far from
being optimal (e.g. mode files were read at every file load or
settings were written at every load or every load causes the
mode config files being read twice).

OTOH, I would like to have that implemented.