Make linux_enable="YES" result in mounting linprocfs, linsysfs,
and handling /dev/shm/.
We used to tell people to symlink /dev/shm to /tmp in pkg-message but some programs check the file system type of /dev/shm (using statfs) so after https://svnweb.freebsd.org/base?view=revision&revision=283461 /dev/shm must be tmpfs for these programs to work properly and /tmp isn't necessarily tmpfs. So I created /compat/linux/dev/shm in https://svnweb.freebsd.org/ports?view=revision&revision=424237. This also makes /dev/shm and /tmp separate namespaces like they are on Linux. The only problem I've encountered with this is when running 'command > /dev/null' as root it will create /compat/linux/dev/null as a regular file. There's no such problem when running commands as a regular user. I think the proper fix is to have a real /dev/shm directory, either by default or created by linux_common.ko. And then let linux_enable mount a tmpfs there.