Fix conststrings type mismatches. (closes #3926)

Fix conststrings type mismatches. (closes #3926)

The generation programs for conststrings.c: txt2c.c and txt2c.sh,
specified the resultant types as "unsigned char[]" while main.c
declared them as "const char[]".

txt2.c generates 0xXX hex codes for each individual character, thus
the "unsigned" definition.  With link-time optimization, some versions
of gcc notice the mismatch and emit a warning.

Change the declarations to match the definitions and cast to char[]
when they are used.