Replace homegrown _seq func with a "for" loop

Bash has a built-in c-style "for" loop, so we don't need to create our
own seq replacement. Portability shouldn't be much of a problem unless
strict POSIX compliance is a requirement. Quoting from
http://wiki.bash-hackers.org/syntax/ccmd/c_for : "C-style for loops
aren't POSIX. They are available in Bash, ksh93, and zsh. All 3 have
essentially the same syntax and behavior."