Improve parsing of program arguments

[tgrez]

When I try to run hatrace for ln -s file1 file2, then I get:

$ hatrace ln -s file1 file2
Invalid option `-s'

Usage: hatrace PROGRAM [PROGRAM_ARG] [--find-nonatomic-writes]
  scriptable strace - trace system calls, signals and more

There is a workaround for this:

hatrace -- ln -s file1 file2

But it would be better to, at least, make it clear in the docs.