Don't call the system toupper or tolower.

Yes, we could cast to unsigned char first, but it's probably safest
to just use our own (in test_util), or remove bad-idea features that
we don't use (in readpassphrase.c).

Fixes 18728.

changes/bug18728

@@ -0,0 +1,4 @@
+  o Minor bugfixes (build):
+    - Resolve warnings when building on systems that are concerned with
+      signed char. Fixes bug 18728; bugfix on 0.2.7.2-alpha and 
+      0.2.6.1-alpha.

src/ext/readpassphrase.c

@@ -142,6 +142,7 @@ restart:
 		p = buf;
 		while ((nr = read(input, &ch, 1)) == 1 && ch != '\n' && ch != '\r') {
 			if (p < end) {
+#if 0
 				if ((flags & RPP_SEVENBIT))
 					ch &= 0x7f;
 				if (isalpha(ch)) {
@@ -150,6 +151,7 @@ restart:
 					if ((flags & RPP_FORCEUPPER))
 						ch = (char)toupper(ch);
 				}
+#endif
 				*p++ = ch;
 			}
 		}

src/ext/tor_readpassphrase.h

@@ -32,9 +32,11 @@
 #define RPP_ECHO_OFF    0x00		/* Turn off echo (default). */
 #define RPP_ECHO_ON     0x01		/* Leave echo on. */
 #define RPP_REQUIRE_TTY 0x02		/* Fail if there is no tty. */
+#if 0
 #define RPP_FORCELOWER  0x04		/* Force input to lower case. */
 #define RPP_FORCEUPPER  0x08		/* Force input to upper case. */
 #define RPP_SEVENBIT    0x10		/* Strip the high bit from input. */
+#endif
 #define RPP_STDIN       0x20		/* Read from stdin, not /dev/tty */
 
 char * readpassphrase(const char *, char *, size_t, int);

src/test/test_util.c

@@ -2297,7 +2297,8 @@ test_util_sscanf(void *arg)
 }
 
 #define tt_char_op(a,op,b) tt_assert_op_type(a,op,b,char,"%c")
-#define tt_ci_char_op(a,op,b) tt_char_op(tolower(a),op,tolower(b))
+#define tt_ci_char_op(a,op,b) \
+  tt_char_op(TOR_TOLOWER((int)a),op,TOR_TOLOWER((int)b))
 
 #ifndef HAVE_STRNLEN
 static size_t