Avoid using the same source and destination in snprintf()

It's undefined to write to a buffer you're reading from in snprintf().
On modern glibc and musl versions, this results in the string
"/zerotier_dump.txt" being generated, i.e. in the root directory. Use a
new variable to hold the string dump.

This is done for MacOS as well. On Sequoia, at least, it's not
necessary, as it handles overlapping objects fine, but this is more
future-proof.

At the same time, include a specific error message when the dumpfile
can't be opened to help users track down problems. Also, truncate the
file so that new writes don't potentially leave stale data.

one.cpp

@@ -1196,12 +1196,13 @@ static int cli(int argc, char** argv)
 			return 0;
 		}
 
-		snprintf((char*)path, sizeof(path), "%s%szerotier_dump.txt", (char*)path, ZT_PATH_SEPARATOR_S);
+		char dumpfile[PATH_MAX];
+		snprintf(dumpfile, sizeof(dumpfile), "%s%szerotier_dump.txt", (char*)path, ZT_PATH_SEPARATOR_S);
 
-		fprintf(stdout, "Writing dump to: %s\n", path);
-		int fd = open((char*)path, O_CREAT | O_RDWR, 0664);
+		fprintf(stdout, "Writing dump to: %s\n", dumpfile);
+		int fd = open(dumpfile, O_CREAT | O_WRONLY | O_TRUNC, 0664);
 		if (fd == -1) {
-			fprintf(stderr, "Error creating file.\n");
+			perror("Error creating file");
 			return 1;
 		}
 		write(fd, dump.str().c_str(), dump.str().size());
@@ -1346,12 +1347,15 @@ static int cli(int argc, char** argv)
 		}
 		close(sock);
 		char cwd[16384];
-		getcwd(cwd, sizeof(cwd));
-		snprintf(cwd, sizeof(cwd), "%s%szerotier_dump.txt", cwd, ZT_PATH_SEPARATOR_S);
-		fprintf(stdout, "Writing dump to: %s\n", cwd);
-		int fd = open(cwd, O_CREAT | O_RDWR, 0664);
+		if (getcwd(cwd, sizeof(cwd)) == nullptr) {
+			strcpy(cwd, ".");
+		}
+		char dumpfile[sizeof(cwd) + 32];
+		snprintf(dumpfile, sizeof(dumpfile), "%s%szerotier_dump.txt", cwd, ZT_PATH_SEPARATOR_S);
+		fprintf(stdout, "Writing dump to: %s\n", dumpfile);
+		int fd = open(dumpfile, O_CREAT | O_WRONLY | O_TRUNC, 0664);
 		if (fd == -1) {
-			fprintf(stderr, "Error creating file.\n");
+			perror("Error creating file");
 			return 1;
 		}
 		write(fd, dump.str().c_str(), dump.str().size());