devfs: add code to mount /dev

All Linux systems need /dev to be set up,so add code to devfs to do
this. The process devfs follows is below.

1. If static_dev is yes, nothing is done.
2. if /dev is an entry in fstab it is mounted or remounted based on that
entry.
3. If /dev is not in fstab, it attempts to mount /dev as a devtmpfs or
   tmpfs depending on which is defined in the kernel; devtmpfs is
   preferred.
4. If neither devtmpfs nor tmpfs is defined, it assumes the user wants
static /dev and prints a warning.

X-Gentoo-Bug: 492694
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=492694