CloseOnExit now defaults to COE_ALWAYS for pterm, bringing it back

CloseOnExit now defaults to COE_ALWAYS for pterm, bringing it back
into line with most other xtermalikes. On Unix, the exit code of a
shell is the last exit code of one of its child processes, even if
it's an interactive shell - so some pterms will close and some will
not for no particularly good reason. Power-detaching a screen
session is especially bad for this.
COE_NORMAL is still useful for specialist purposes (running a single
command in its own pterm), but I don't think it's a sane default,
unfortunately.

git-svn-id: svn://svn.tartarus.org/sgt/putty@2154 cda61777-01e9-0310-a592-d414129be87e