Fix DX_COMMAND_OUTPUT backslash-newline on heirloom-sh.

It seems that heirloom-sh (and presumably Solaris /bin/sh) read just
silently eats backslash-newlines, and escaping the backslash does
nothing useful, the shell just eats both backslashes and the newline.

Attempt to avoid this problem by appending a magic string to
troublesome lines, and stripping it after reading.  This will
obviously fail if the input _actually_ has this sequence but
hopefully that will not be an issue in practice.