kernel: Cleanup <sys/uio.h> issues.

kernel: Cleanup <sys/uio.h> issues.

 The iovec_free() inline very complicates this header inclusion.  The
 NULL check is not always seen from <sys/_null.h>.  Luckily only three
 kernel sources needs it: kern_subr.c, sys_generic.c and uipc_syscalls.c.
 Also just a single dev/drm source makes use of 'struct uio'.
 * Include <sys/uio.h> explicitly first in drm_fops.c to avoid kfree()
   macro override in drm compat layer.
 * Use <sys/_uio.h> where only enums and struct uio is needed, but ensure
   that userland will not include it for possible later <sys/user.h> use.
 * Stop using <sys/vnode.h> as shortcut for uiomove*() prototypes.  The
   uiomove*() family functions possibly transfer data across kernel/user
   space boundary.  This header presence explicitly mark sources as such.
 * Prefer to add <sys/uio.h> after <sys/systm.h>, but before <sys/proc.h>
   and definitely before <sys/malloc.h> (except for 3 mentioned sources).
   This will allow to remove <sys/malloc.h> from <sys/uio.h> later on.
 * Adjust <sys/user.h> to use component headers instead of <sys/uio.h>.

 While there, use opportunity for a minimal whitespace cleanup.

 No functional differences observed in compiler intermediates.