libthread: use devdup instead of mounting pipe to /mnt/temp for close-on-exec in...

libthread: use devdup instead of mounting pipe to /mnt/temp for close-on-exec in procexec()

the namespace might be shared by other processes. instead, we
create a anonymous pipe with pipe() and use devdup to open one
end close-on-exec. this is shorter and avoids the race condition.

do not touch Execargs after writing the error message as the
process might be gone after the write. this was to manually
close the fd which isnt neccesary as the kernel will do it
for us on the following exit.