Redmine #2704: fix canonify issue with negative signed chars

On HP-UX 11.11, the "isalnum()" call expects a range of values from
-1 to 255. However the range of a signed char is -127 to 127, and
when an out-of-range negative number is passed to isalnum() on this
platform, the results in the case of the Yen symbol in acceptance
test 01_vars/02_functions/007.cf, with a signed char value of -91,
isalnum() returns "2" which is taken to mean that the Yen symbol
is alphanumeric.

By casting to "unsigned char" before the cast to "int" for the call,
the "-91" value is converted to a valid character set value of 165,
which is properly handled by isalnum() and thus allows the test to
pass.
Merge pull request #681 from mvpel/redmine_2704

Redmine #2704: fix canonify issue with negative signed chars