vsprintf: factorize "(null)" string

This patchset reduces lib/lib.a code size by 482 bytes on my Core 2 with
gcc 4.4.1 even considering that it exports a newly defined function
skip_spaces() to drivers:

   text    data     bss     dec     hex filename
  64867     840     592   66299   102fb (TOTALS-lib.a-BEFORE)
  64641     584     592   65817   10119 (TOTALS-lib.a-AFTER)
and implements some code tidy up.

Besides reducing lib.a size, it converts many in-tree drivers to use the
newly defined function, which makes another small reduction on kernel size
overall when those drivers are used.

This patch:

Change "<NULL>" to "(null)", unifying 3 equal strings.
glibc also uses "(null)" for the same purpose.

It decreases code size by 7 bytes:
 text    data     bss     dec     hex filename
15765       0       8   15773    3d9d vsprintf.o (ex lib/lib.a-BEFORE)
15758       0       8   15766    3d96 vsprintf.o (ex lib/lib.a-AFTER)

Signed-off-by: André Goddard Rosa <andre.goddard@gmail.com>
Acked-by: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>