compat: fix use of <ctype.h> functions

Unfortunately, the C standard requires an 'unsigned char' value or EOF.
So we cannot blindly pass a 'char' value, which may be signed depending
on the architecture.

compat/strcasecmp.c

@@ -33,10 +33,11 @@ int strcasecmp (const char *s1, const char *s2)
 #else
     for (size_t i = 0;; i++)
     {
-        int d = tolower (s1[i]) - tolower (s2[i]);
-        if (d || !s1[i])
+        unsigned char c1 = s1[i], c2 = s2[i];
+        int d = tolower (c1) - tolower (c2);
+        if (d || !c1)
             return d;
-        assert (s2[i]);
+        assert (c2);
     }
 #endif
 }

compat/strcasestr.c

@@ -34,17 +34,17 @@ char *strcasestr (const char *psz_big, const char *psz_little)
 
     while( *p_pos )
     {
-        if( toupper( *p_pos ) == toupper( *psz_little ) )
+        if( toupper( (unsigned char)*p_pos ) == toupper( (unsigned char)*psz_little ) )
         {
-            char * psz_cur1 = p_pos + 1;
-            char * psz_cur2 = (char *)psz_little + 1;
-            while( *psz_cur1 && *psz_cur2 &&
-                   toupper( *psz_cur1 ) == toupper( *psz_cur2 ) )
+            char *cur1 = p_pos + 1;
+            char *cur2 = (char *)psz_little + 1;
+            while( *cur1 && *cur2
+             && toupper( (unsigned char)*cur1 ) == toupper( (unsigned char)*cur2 ) )
             {
-                psz_cur1++;
-                psz_cur2++;
+                cur1++;
+                cur2++;
             }
-            if( !*psz_cur2 ) return p_pos;
+            if( !*cur2 ) return p_pos;
         }
         p_pos++;
     }

compat/strncasecmp.c

@@ -33,10 +33,11 @@ int strncasecmp (const char *s1, const char *s2)
 #else
     for (size_t i = 0; i < n; i++)
     {
-        int d = tolower (s1[i]) - tolower (s2[i]);
-        if (d || !s1[i])
+        unsigned char c1 = s1[i], c2 = s2[i];
+        int d = tolower (c1) - tolower (c2);
+        if (d || !c1)
             return d;
-        assert (s2[i]);
+        assert (c2);
     }
     return 0;
 #endif