Improve invocation compatibility with awk

[dspinellis]

At a minimum we should support this style of invocation

awk [-n N] [-d[n]] [-F fs] [-v var=value] [prog | -f progfile] [file ...]

• -F fs Define the input field separator to be the regular expression fs.
• -v var=value Assign value to variable var before prog is executed; any number of -v options may be present.
• -f progfile Read program code from the specified file progfile instead of from the command line.
• -n n Specify the number of threads to use. When n is zero (or not specified), the number of threads will equal the number of available cores.

Note that multiple input files can be specified.

If no input file is specified, then awk should read from its standard input.

Use Go getopt for processing arguments.