refactor(fileio): remove API shell layer encouraging unnecessary allocations

Functions like file_open_new() and file_open_fd_new() which just is a
wrapper around the real functions but with an extra xmalloc/xfree around
is an anti-pattern. If the caller really needs to allocate a
FileDescriptor as a heap object, it can do that directly.

FileDescriptor by itself is pretty much a pointer, or rather two:
the OS fd index and a pointer to a buffer. So most of the time an extra
pointer layer is just wasteful.

In the case of scriptin[curscript] in getchar.c, curscript used
to mean in practice:

N+1 open scripts           when curscript>0
zero or one open scripts   when curscript==0

Which means scriptin[0] had to be compared to NULL to disambiguate the
curscript=0 case.

Instead, use curscript==-1 to mean that are no script,
then all pointer comparisons dissappear and we can just use an array of
structs without extra pointers.

src/nvim/lua/executor.c

@@ -1828,7 +1828,11 @@ bool nlua_exec_file(const char *path)
     lua_getglobal(lstate, "loadfile");
     lua_pushstring(lstate, path);
   } else {
-    FileDescriptor *stdin_dup = file_open_stdin();
+    FileDescriptor stdin_dup;
+    int error = file_open_stdin(&stdin_dup);
+    if (error) {
+      return false;
+    }
 
     StringBuilder sb = KV_INITIAL_VALUE;
     kv_resize(sb, 64);
@@ -1837,7 +1841,7 @@ bool nlua_exec_file(const char *path)
       if (got_int) {  // User canceled.
         return false;
       }
-      ptrdiff_t read_size = file_read(stdin_dup, IObuff, 64);
+      ptrdiff_t read_size = file_read(&stdin_dup, IObuff, 64);
       if (read_size < 0) {  // Error.
         return false;
       }
@@ -1849,7 +1853,7 @@ bool nlua_exec_file(const char *path)
       }
     }
     kv_push(sb, NUL);
-    file_free(stdin_dup, false);
+    file_close(&stdin_dup, false);
 
     lua_getglobal(lstate, "loadstring");
     lua_pushstring(lstate, sb.items);