Use symlinks for kernel modules rather than hardlinks

When aliasing a kernel module to a different name (ie if_igb for if_em),
it's better to use symlinks than hard links. kldxref will omit entries for
the links, ensuring that the loaded module has the correct name.

Reviewed by: imp
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D19979