ssh-askpass-darwin: style fixups

Signed-off-by: David Aguilar <davvid@gmail.com>
ssh-askpass-darwin: redirect osascript errors to /dev/null

Signed-off-by: David Aguilar <davvid@gmail.com>
ssh-askpass-darwin: account for newer versions of Mac OS X

The osascript snippet we use to prompt the user for their password has
different output on Mac OS X 10.9 Mavericks (and possibly others).

The output can start with either:
	"button returned:OK, "
or
	"text returned:<password>".

We were only accounting for the latter form.  Account for the new-style
output with an additional sed expression.

Signed-off-by: David Aguilar <davvid@gmail.com>