* moddir:
*
* Set the search path for modules. This has a format similar to the
* csh path variable. If the module isn't found in the first directory
* it tries the second and so on. The default is /kernel /usr/kernel
*
* Example:
* moddir: /kernel /usr/kernel /other/modules
通過使用moddir: 搜索模塊目錄列表，可以修改默認的路經
* root device and root filesystem configuration:
*
* The following may be used to override the defaults provided by
* the boot program:
*
* rootfs: Set the filesystem type of the root.
* 用來標志根文件系統(tǒng)類型
* rootdev: Set the root device. This should be a fully
* expanded physical pathname. The default is the
* physical pathname of the device where the boot
* program resides. The physical pathname is
* highly platform and configuration dependent.
* 用來標示根設備和引導程序駐留的設備不同（指定根設備）
* Example:
* rootfs:ufs
* rootdev:/sbus@1,f8000000/esp@0,800000/sd@3,0:a
*
* (Swap device configuration should be specified in /etc/vfstab.)
* exclude:
*
* Modules appearing in the moddir path which are NOT to be loaded,
* even if referenced. Note that `exclude' accepts either a module name,
* or a filename which includes the directory.
*
* Examples:
* exclude: win
* exclude: sys/shmsys
不再使用的模塊通過exclude: 來排除
* forceload:
*
* Cause these modules to be loaded at boot time, (just before mounting
* the root filesystem) rather than at first reference. Note that
* forceload expects a filename which includes the directory. Also
* note that loading a module does not necessarily imply that it will
* be installed.
*
* Example:
* forceload: drv/foo
使用forceload: 需要在初始化時強制加載的模塊
* set:
*
* Set an integer variable in the kernel or a module to a new value.
* This facility should be used with caution. See system(4).
*
* Examples:
*
* To set variables in 'unix':
*
* set nautopush=32
* set maxusers=40
*
* To set a variable named 'debug' in the module named 'test_module'
*
* set test_module:debug = 0x13