Turn on resolving of "@" character to vimage name on per-vimage

Turn on resolving of "@" character to vimage name on per-vimage
basis via vfs.morphing_symlinks sysctl.  This finally allows
IMUNES to be run on top of FreeBSD -CURRENT.

NB as of now multicast sending without having default route set still
seems to be broken in FreeBSD -CURRENT (as it is in FreeBSD 5 and 6
as well), so until this is resolved in the kernel or we introduce some
other hack, routing protocols / implementations relying on multicast
transmission won't work inside IMUNES.

Bug found by:
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