/etc/rc.d/mdconfig supports mdconfig unit name detection.

(line 194)
if [ -z "${_mdconfig_list}" ]; then
        for _mdconfig_config in `list_vars mdconfig_md[0-9]\* |

However, it treats all md[0-9]* as mdconfig unit even for extra parameters.
For example, "mdconfig_md0_perms" will be recognized as unit "md0_perms".

This will produce error messages during boot. But seems harmless.

How-To-Repeat: mdconfig_md0="-t swap -s 512m"
mdconfig_md0_perms="1777"

Following is boot error message:

Creating md0 device (swap).
Mounting /dev/md0.
Creating md0_perms device (1777).
usage: mdconfig -a -t type [-n] [-o [no]option] ... [-f file]
                [-s size] [-S sectorsize] [-u unit]
                [-x sectors/track] [-y heads/cylinder]
       mdconfig -d -u unit [-o [no]force]
       mdconfig -r -u unit -s size [-o [no]force]
       mdconfig -l [-v] [-n] [-f file] [-u unit]
       mdconfig file
           type = {malloc, vnode, swap}
           option = {cluster, compress, reserve}
           size = %d (512 byte blocks), %db (B),
                  %dk (kB), %dm (MB), %dg (GB) or
                  %dt (TB)
Creating md0_perms device failed, moving on.