grep -R hangs if applied upon directory that contains pipe file

[Elmar Stellnberger]

grep -R should only open regular files, no pipes, no devices, because
otherwise grep -R will hang on these files.

i.e. > grep -R seekitem .
is expected to behave like
> find . -type f -exec grep seekitem {} +

reproduce by:
> mkfifo hug
> grep -R seek .