zdtm: fallback to mknod(/dev/ptmx) when CONFIG_DEVPTS_MULTIPLE_INSTANCES is not set
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 <46f5a0051bcaac06293374f80c77669d4919a80b@google.com>
Acked-by: Andrew Vagin <74c05afa918858348d6db8037d82fa8124870113@openvz.org>
Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@parallels.com>

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