Add isblank

Existing ctype checks are implemented using a 256 byte lookup table,
allowing each character to be in any of 8 character classes. Since there
are 8 existing character classes without the blank class, I implemented
isblank without using the lookup table.  Since there are only two blank
characters - tab and space - this is a more reasonable approach than
doubling the size of the lookup table to accommodate one more class.

Signed-off-by: Jason Hobbs <jason.hobbs@calxeda.com>