dos: improve MBR signature generation

Using tv_usec in struct timeval from gettimeofday() doesn't provide
enough precision to fill an unsigned 32-bit integer and isn't really
random. It it always less than one million when using the GNU C library
while an unsigned 32-bit integer ranges between 0 and 4294967295.

In FAT filesystem creation, parted already uses a better random
generator, so move that code into a common function and use it
for MS-DOS MBR signature generation.

* libparted/fs/r/fat/fat.c (_gen_new_serial_number): Remove.
(fat_create): Use generate_random_uint32 instead of
_gen_new_serial_number.
* libparted/labels/dos.c (generate_random_id): Remove.
(msdos_write): Use generate_random_uint32 instead of
generate_random_id.
* libparted/labels/misc.h (generate_random_uint32): New function.
Created from _gen_new_serial_number in libparted/fs/r/fat/fat.c with
additional check to avoid returning zero, which may be interpreted
as no FAT serial number or no MBR signature.
bootstrap: update to latest from gnulib