If pterm's execvp fails when given the whole argument list after -e,

If pterm's execvp fails when given the whole argument list after -e,
and the argument list contains only one string, try again by passing
that single string to "$SHELL -c" to be parsed as a shell command.
This matches xterm's behaviour (as of xterm 261, at least), and means
in practice that users can do _either_ of 'pterm -e some command' and
'pterm -e "some command"'.

(A quick survey suggests that the majority of X terminal programs agree
with pterm's old behaviour of only supporting '-e some command',
except that gnome-terminal only supports the other behaviour and xterm
supports both. With that disagreement, I think supporting both is
probably the sensible thing.)

git-svn-id: svn://svn.tartarus.org/sgt/putty@9575 cda61777-01e9-0310-a592-d414129be87e