Bug fix

• use libptytty by default and add --with-libptytty option to configure. Keep the original ptytty.c code as a fallback.

• add libtinfow to the list of libraries that are checked for the presence of tgetent()

• only look for filters in $RLWRAP_FILTERDIR and don't add this directory to filter's PATH

• have rlwrap source conform to POSIX.1c (as we cannot use Polarhome anymore to test on ancient systems)

• extend testclient with a test that spawns a child and then dies (testing the effect of --skip-setctty)

• make configure backdate src/completion.rb a few seconds to prevent spurious calls to rbgen