zdtm: fallback to mknod(/dev/ptmx) when CONFIG_DEVPTS_MULTIPLE_INSTANCES is not set

A kernel without that option configured does not have /dev/pts/ptmx, so
fallback to the previous way of creating it using mknod instead.

The previous code was trying to bind mount ptmx on top of a symlink, which does
not actually work... Keep only the symlink call and use a relative symlink
instead. Adjust the error message of the symlink case to mention symlink()
instead of mknod() and also /dev/ptmx instead of /dev/pts.

Tested:
- zdtm test suite runs on ^ns/static/.* before and after the change.
- Same on a kernel with CONFIG_DEVPTS_MULTIPLE_INSTANCES unset.

Signed-off-by: Filipe Brandenburger <filbranden@google.com>
Acked-by: Andrew Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>