Support large bdevs on 32-bit; MB units by default.

Change all instances of "unsigned long" where referring to a bdev size
to uint64_t; this fixes some overflows on 32-bit machines, where
"unsigned long" is uint32_t. Support all unit-sizes supported by LVM
except 's' and 'e' [bkmgt]. Print a warning and use default bdev-size if
invalid unit-size specified.

Signed-off-by: Stephen Ayotte <stephen.ayotte@gmail.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>