Be more careful about signed vs. unsigned char

The buildfarm has reminded me that not all systems consider char to be
signed and we need to be explicit.  Adjust the various bits of mac8.c
for what we intend, mostly using casts to unsigned char as suggested by
Tom, and adjust the tests for valid input accordingly.  Explicitly make
the hexlookup table signed as it's useful to use -1 there to indicate an
invalid value.

src/backend/utils/adt/mac8.c

@@ -35,9 +35,9 @@
 #define lobits(addr) \
   ((unsigned long)(((addr)->e<<24) | ((addr)->f<<16) | ((addr)->g<<8) | ((addr)->h)))
 
-static unsigned char hex2_to_uchar(const char *str, const char *ptr);
+static unsigned char hex2_to_uchar(const unsigned char *str, const unsigned char *ptr);
 
-static const char hexlookup[128] = {
+static const signed char hexlookup[128] = {
 	-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
 	-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
 	-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
@@ -58,16 +58,16 @@ static const char hexlookup[128] = {
  * the entire string, which is used only for error reporting.
  */
 static inline unsigned char
-hex2_to_uchar(const char *ptr, const char *str)
+hex2_to_uchar(const unsigned char *ptr, const unsigned char *str)
 {
 	unsigned char ret = 0;
-	char		lookup;
+	signed char lookup;
 
 	/* Handle the first character */
-	if (*ptr < 0)
+	if (*ptr > 127)
 		goto invalid_input;
 
-	lookup = hexlookup[(unsigned char) *ptr];
+	lookup = hexlookup[*ptr];
 	if (lookup < 0)
 		goto invalid_input;
 
@@ -76,10 +76,10 @@ hex2_to_uchar(const char *ptr, const char *str)
 	/* Move to the second character */
 	ptr++;
 
-	if (*ptr < 0)
+	if (*ptr > 127)
 		goto invalid_input;
 
-	lookup = hexlookup[(unsigned char) *ptr];
+	lookup = hexlookup[*ptr];
 	if (lookup < 0)
 		goto invalid_input;
 
@@ -103,8 +103,8 @@ invalid_input:
 Datum
 macaddr8_in(PG_FUNCTION_ARGS)
 {
-	const char *str = PG_GETARG_CSTRING(0);
+	const unsigned char *str = (unsigned char*) PG_GETARG_CSTRING(0);
-	const char *ptr = str;
+	const unsigned char *ptr = str;
 	macaddr8   *result;
 	unsigned char a = 0,
 				b = 0,
@@ -115,10 +115,10 @@ macaddr8_in(PG_FUNCTION_ARGS)
 				g = 0,
 				h = 0;
 	int			count = 0;
-	char		spacer = '\0';
+	unsigned char spacer = '\0';
 
 	/* skip leading spaces */
-	while (*ptr && isspace((unsigned char) *ptr))
+	while (*ptr && isspace(*ptr))
 		ptr++;
 
 	/* digits must always come in pairs */
@@ -191,9 +191,9 @@ macaddr8_in(PG_FUNCTION_ARGS)
 		/* allow trailing whitespace after if we have 6 or 8 bytes */
 		if (count == 6 || count == 8)
 		{
-			if (isspace((unsigned char) *ptr))
+			if (isspace(*ptr))
 			{
-				while (*++ptr && isspace((unsigned char) *ptr));
+				while (*++ptr && isspace(*ptr));
 
 				/* If we found a space and then non-space, it's invalid */
 				if (*ptr)