Build warnings with isXXXX() function/macros

Hi,

the build of OpenDNSSEC 1.4.9 produces a few warnings in the build log:

strlcat.c:63:0: warning: ISO C forbids an empty translation unit
strlcpy.c:59:0: warning: ISO C forbids an empty translation unit
b64_pton.c:154:3: warning: array subscript has type 'char'
libhsm.c:296:28: warning: ISO C forbids conversion of object pointer to function pointer type
datetime.c:687:9: warning: array subscript has type 'char'
string_util2.c:599:13: warning: array subscript has type 'char'

I'll let you folks deal with the libhsm.c warning. It seems to be difficult
to deal with, CK_C_GetFunctionList appears to be declared as a function
pointer, and dlsym() returns void*.

The "no empty translation unit" is probably because this platform has
the associated functions natively.

However, the last class can be potentially nasty/unportable:
the isXXXX() macros/functions only accept the value range of
unsigned char and -1 (or EOF? I forget) as arguments, and anything
else results in undefined behavior if I recall correctly.

On most platforms, "char" is actually "signed char" and can take
values outside of the domain for these functions.

Therefore: as long as EOF is dealt with, the proper way to deal with
these is either to use "unsigned char" as variable type, or cast the
arguments to the isXXXX() functions/macros to "unsigned char".