Windows: Implement start_command().

On Windows, we have spawnv() variants to run a child process instead of
fork()/exec(). In order to attach pipe ends to stdin, stdout, and stderr,
we have to use this idiom:

save1 = dup(1);
dup2(pipe[1], 1);
spawnv();
dup2(save1, 1);
close(pipe[1]);

assuming that the descriptors created by pipe() are not inheritable.

Signed-off-by: Johannes Sixt <johannes.sixt@telecom.at>