rc: _var_mfs and _var_script handling

[fichtner]

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• ~~_var_mfs can be removed along with the glue to move/shift directories in and out of it~~
• ~~_var_script can be renamed, maybe we can use _precmd~~ ~~add canonical _setup and use it~~ https://github.com/opnsense/src/commit/3edcfbc578f
• ~~upstream _setup to freebsd~~
• ~~remove _var_script handing in rc.freebsd~~
• ~~support NAME_setup for NAME_reload~~ https://github.com/opnsense/src/commit/c7fed157a7721
• convert plugins setup.sh as well once in the base system