mark <zlib.h> with for-crc32 in a consistent manner

mark <zlib.h> with for-crc32 in a consistent manner

in many cases, <zlib.h> is only included for crc32 function,
and in some of them, there's a comment saying that, but in
a different way.  In one place (hw/net/rtl8139.c), there was
another #include added between the comment and <zlib.h> include.

Make all such comments to be on the same line as #include, make
it consistent, and also add a few missing comments, including
hw/nvram/mac_nvram.c which uses adler32 instead.

There's no code changes.

Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>