mtd-utils: nanddump/nandwrite: style, signed-ness, printing fixups

mtd-utils: nanddump/nandwrite: style, signed-ness, printing fixups

There were some signed/unsigned integer comparisons. Their types were
changed for safety. Also, "strtol" was improperly used for unsigned
data types.

Nanddump's pretty print options needed a slight reformat to prepare for
printing offsets that are more than 32 bits (8 hex characters) wide.
This prevents overlap of output and ensures that at least one space is
printed between hex and ascii printouts. Perhaps this could use some
better alignment in the future.

Other fixes:
* Corrected several simple spacing issues
* Changed indentation of some global variable declarations in
  order to prepare for the next patch, which makes those
  declarations longer
* Used macro for PRETTY_ROW_SIZE instead of constant 16
* Reformatted, edited a multi-line comment
* Removed some unnecessary casts

Signed-off-by: Brian Norris <computersforpeace@gmail.com>