Fix some compiler warnings

Fix compiler warnings about comparing signed with unsigned integers.

src/hdlc.c

@@ -199,7 +199,7 @@ ssize_t hdlc_find_frame(const uint8_t *buffer, size_t bufsize, off_t *start)
 	int s = -1, e = -1;
 
 	// Look for frame start
-	for (int i = *start; i < bufsize; i++) {
+	for (size_t i = *start; i < bufsize; i++) {
 		if (buffer[i] == 0x7e) { // Flag Sequence
 			s = i + 1;
 			break;
@@ -213,7 +213,7 @@ ssize_t hdlc_find_frame(const uint8_t *buffer, size_t bufsize, off_t *start)
 		s++;
 
 	// Look for frame end
-	for (int i = s; i < bufsize; i++) {
+	for (size_t i = s; i < bufsize; i++) {
 		if (buffer[i] == 0x7e) { // Flag Sequence
 			e = i;
 			break;

src/ipv4.c

@@ -1052,7 +1052,7 @@ int ipv4_restore_routes(struct tunnel *tunnel)
 
 static inline char *replace_char(char *str, char find, char replace)
 {
-	for (int i = 0; i < strlen(str); i++)
+	for (size_t i = 0; i < strlen(str); i++)
 		if (str[i] == find)
 			str[i] = replace;
 	return str;

src/tunnel.c

@@ -773,7 +773,7 @@ static int tcp_connect(struct tunnel *tunnel)
 		const char *response = NULL;
 
 		memset(&(request), 0, sizeof(request));
-		for (int j = 0; response == NULL; j++) {
+		for (unsigned int j = 0; response == NULL; j++) {
 			if (j >= ARRAY_SIZE(request) - 1) {
 				log_error("Proxy response is unexpectedly large and cannot fit in the %lu-bytes buffer.\n",
 				          ARRAY_SIZE(request));
@@ -819,7 +819,7 @@ static int tcp_connect(struct tunnel *tunnel)
 				};
 				const char *eol = NULL;
 
-				for (int i = 0; (i < ARRAY_SIZE(HTTP_EOL)) &&
+				for (unsigned int i = 0; (i < ARRAY_SIZE(HTTP_EOL)) &&
 				     (eol == NULL); i++)
 					eol = strstr(response, HTTP_EOL[i]);
 				response = eol;

src/userinput.c

@@ -332,7 +332,7 @@ void read_password(const char *pinentry, const char *hint,
 {
 	int masked = 0;
 	struct termios oldt, newt;
-	int i;
+	size_t i;
 
 	if (pinentry && *pinentry) {
 		pinentry_read_password(pinentry, hint, prompt, pass, len);