Add back in a scheme to emulate old type major/minor numbers via hooks into
stat, linprocfs to return major/minors that Linux app's expect.  Currently
only /dev/null is always registered.  Drivers can register via the Linux
type shim similar to the ioctl shim but by using
linux_device_register_handler/linux_device_unregister_handler functions.
The structure is:

    struct linux_device_handler {
        char    *bsd_driver_name;
        char    *linux_driver_name;
        char    *bsd_device_name;
        char    *linux_device_name;
        int     linux_major;
        int     linux_minor;
        int     linux_char_device;
    };

Linprocfs uses this to display the major number of the driver.  The
soon to be available linsysfs will use it to fill in the driver name.
Linux_stat uses it to translate the major/minor into Linux type values.

Note major numbers are dynamically assigned via passing in a -1 for
the major number so we don't need to keep track of them.

This is somewhat needed due to us switching to our devfs.  MegaCli
will not run until I add in the linsysfs and mfi Linux compat changes.

Sponsored by:	IronPort Systems

Tested by:	cross-compile

Pointyhat to:	netchild

Submitted by:	Divacky Roman <xdivac02@stud.fit.vutbr.cz>
Obtained from:	DragonFly (some parts)

  the callers if the exec either succeeds or fails early.
- Move the code to call exit1() if the exec fails after the vmspace is
  gone to the bottom of kern_execve() to cut down on some code duplication.