Add headers to Extra-Source-Files, fixes #2.
get rid of capi (https://ghc.haskell.org/trac/ghc/ticket/7388)
disable CApiFFI
support for linux; added a test ptywrap executable
cabal file: add default-langage for exe:ptywrap
handle linux' weird tty error codes
pids!
update ptywrap to not do buffering and wait for the child process
put includes into extra-source-files, fixes #2