LP backend should not use system

[dra27]

The use of the system call to invoke the external LP solver in the LP backend exposes the command to shell escaping madness (especially on Windows, where the implementation in #7 correctly reads TMP/TEMP).

Having eliminated the use of cat and tee in #8, it should be possibly to invoke the executable directly using posix_spawn (or equivalent), thus mediating the nightmare of shell escaping.