Remove use of set_rcvar in rc.d startup scripts.

In FreeBSD, set_rcvar() was removed here:

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    |r230103 | dougb | 2012-01-14 00:59:02 -0800 (Sat, 14 Jan 2012) | 6 lines
    |
    |Now that its callers have been udpated, remove set_rcvar().
    |
    |The concept of set_rcvar() was nice in theory, but the forks
    |it creates are a drag on the startup process, which is especially
    |noticeable on slower systems, such as embedded ones.
    |
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Perform similar fixes to the FreeNAS startup scripts to accomodate this change
in FreeBSD, and future-proof our startup scripts when we upgrade
FreeBSD code in FreeNAS:

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    |r230099 | dougb | 2012-01-13 18:18:41 -0800 (Fri, 13 Jan 2012) | 14 lines
    |
    |Prepare for the removal of set_rcvar() by changing the rcvar=
    |assignments to the literal values it would have returned.
    |
    |The concept of set_rcvar() was nice in theory, but the forks
    |it creates are a drag on the startup process, which is especially
    |noticeable on slower systems, such as embedded ones.
    |
    |During the discussion on freebsd-rc@ a preference was expressed for
    |using ${name}_enable instead of the literal values. However the
    |code portability concept doesn't really apply since there are so
    |many other places where the literal name has to be searched for
    |and replaced. Also, using the literal value is also a tiny bit
    |faster than dereferencing the variables, and every little bit helps.
    |
    |------------------------------------------------------------------------