Clean up the code around isatty() a bit

src/fio.cpp

@@ -1,5 +1,7 @@
 #include "fio.hpp"
 
+#include <unistd.h>
+
 #include <cstdlib>
 
 Result<String> read_fh(FILE* file) {
@@ -37,3 +39,7 @@ Result<String> read_file(const char* filename) {
 }
 
 Result<String> read_stdin() { return read_fh(stdin); }
+
+// TODO: though unlikely, but isatty can possibly set an error. Need to check it
+// someday.
+bool stdin_isatty() { return isatty(STDIN_FILENO); }

src/fio.hpp

@@ -5,3 +5,4 @@
 Result<String> read_file(const char* filename);
 
 Result<String> read_stdin();
+bool stdin_isatty();

src/vli.cpp

@@ -1,6 +1,3 @@
-#include <unistd.h>
-
-#include <cstdio>
 #include <iostream>
 
 #include "arena.hpp"
@@ -17,7 +14,7 @@ StaticArena<64 * 1024 * 1024> arena;
 Result<void> run(int argc, const char* argv[]) {
   String src = DIEX(String::create(""));
   if (argc == 1) {
-    if (isatty(STDIN_FILENO)) {
+    if (stdin_isatty()) {
       die("Code expected at stdin, not a tty.\n");
     }
     src = TRY(read_stdin());