parsing: fix header typo
filter: make exit status local

It's only used in one place, and not useful to have around since
close_filter will die() if exit_status isn't what it expects, anyway. So
this is best as just a local variable instead of as part of the struct.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
filter: split filter functions into their own file

A first step for more interesting things.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>