Send EOF when closing first, before SIGHUP

According to the bash maintainer sending EOF is the appropriate way of
telling e. g. bash to quit:
    http://permalink.gmane.org/gmane.comp.shells.bash.bugs/12602

I wasn't able to get KPtyProcess to send that in a nice way (calling
setStandardInputFile(QProcess::nullDevice(), for example), so we fetch
the EOF character manually with tcgetattr and send that.

BUG: 185140
REVIEW: 128416