Switch the string shootouts to having main take an argument

The Computer Language Benchmarks Game treats all its benchmarks the same in its
bencher script by sending in their size to the executable.  However, the string
shootouts (Regex DNA, Reverse Complement, and K-nucleotide) require an input
file instead of a problem size.  To get around the bogus size value sent to
them, we previously had a similarly bogus config variable, to match the set up
for our other benchmarks.  However, it seems somewhat cleaner to make use of our
command line argument syntax and then ignore the argument in main itself.