sfeed_curses: close stdin for spawning a plumb program in non-interactive mode

This is only for plumbing in non-interactive mode in forkexec(), but not piping
content.

Probably obvious what the descriptors are, but also add a few comments to dup2
of the file descriptors (stdin, stdout, stderr).

To reproduce a behaviour:

	Plumb script:

	#!/bin/sh
	dmenu

	Then launch it:

	SFEED_PLUMB_INTERACTIVE=0 SFEED_PLUMB=thescript sfeed_curses ~/.sfeed/feeds/*

The program now waits on input while in non-interactive mode and only seems to
hang.

After:

The program starts but just has no input passed to it.

sfeed_curses.1

@@ -1,4 +1,4 @@
-.Dd May 4, 2022
+.Dd May 5, 2022
 .Dt SFEED_CURSES 1
 .Os
 .Sh NAME
@@ -290,6 +290,7 @@ In non-interactive mode
 .Nm
 doesn't wait until the process exits.
 Stdout and stderr of the program are not written as output.
+When plumbing an URL then stdin is closed also.
 .Sh EXIT STATUS
 .Ex -std
 The exit status is 130 on SIGINT and 143 on SIGTERM.

sfeed_curses.c

@@ -604,8 +604,8 @@ pipeitem(const char *cmd, struct item *item, int field, int interactive)
 		die("fork");
 	case 0:
 		if (!interactive) {
-			dup2(devnullfd, 1);
-			dup2(devnullfd, 2);
+			dup2(devnullfd, 1); /* stdout */
+			dup2(devnullfd, 2); /* stderr */
 		}
 
 		errno = 0;
@@ -642,8 +642,9 @@ forkexec(char *argv[], int interactive)
 		die("fork");
 	case 0:
 		if (!interactive) {
-			dup2(devnullfd, 1);
-			dup2(devnullfd, 2);
+			dup2(devnullfd, 0); /* stdin */
+			dup2(devnullfd, 1); /* stdout */
+			dup2(devnullfd, 2); /* stderr */
 		}
 		if (execvp(argv[0], argv) == -1)
 			_exit(1);
@@ -1847,8 +1848,8 @@ markread(struct pane *p, off_t from, off_t to, int isread)
 	case -1:
 		die("fork");
 	case 0:
-		dup2(devnullfd, 1);
-		dup2(devnullfd, 2);
+		dup2(devnullfd, 1); /* stdout */
+		dup2(devnullfd, 2); /* stderr */
 
 		errno = 0;
 		if (!(fp = popen(cmd, "w")))