fix possible DoS

The regex to detect AWK files contained two star operators, which are easily
exploited to cause excessive backtracking in the regex engine. Instead, limit
to 100 repetitions. Obviously, this is a compromise, e.g. an AWK file with 101
leading spaces will no longer be detected.

See also http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=571038 , since Debian had that awk expression, first.