Check for symlinks before attempting create.

When attempting to create the compulsory symlinks in /dev,
check for the existence of the link using stat first before
blindly attempting to create the link.

This works around an apparent quirk in the kernel VFS on read-only
file systems where the returned error code might be EEXIST or EROFS
depending on previous access to the /dev directory and its entries.
Reported-by: William Dauchy <william@gandi.net>
Signed-off-by: Michael H. Warfield <mhw@WittsEnd.com>
Tested-by: William Dauchy <william@gandi.net>