Support larger RADIUS attributes in libkrad

Support larger RADIUS attributes in libkrad

In kr_attrset_decode(), explicitly treat the length byte as unsigned.
Otherwise attributes longer than 125 characters will be rejected with
EBADMSG.

Add a 253-character-long NAS-Identifier attribute to the tests to make
sure that attributes with the maximal number of characters are working
as expected.

[ghudson@mit.edu: used uint8_t cast per current practices; edited
commit message]

ticket: 9036 (new)