pager: add a trivial internal pager

In the very unlikely scenario where none of the external pagers is
available, use an internal implementation to pass stdin to stdout.

Don't bother with trying 'cat', because it's no more useful than the
internal pager.

https://bugzilla.redhat.com/show_bug.cgi?id=713707

src/pager.c

@@ -20,6 +20,7 @@
 ***/
 
 #include <sys/types.h>
+#include <fcntl.h>
 #include <stdlib.h>
 #include <unistd.h>
 #include <string.h>
@@ -31,6 +32,18 @@
 
 static pid_t pager_pid = 0;
 
+static void pager_fallback(void) {
+        ssize_t n;
+        do {
+                n = splice(STDIN_FILENO, NULL, STDOUT_FILENO, NULL, 64*1024, 0);
+        } while (n > 0);
+        if (n < 0) {
+                log_error("Internal pager failed: %m");
+                _exit(EXIT_FAILURE);
+        }
+        _exit(EXIT_SUCCESS);
+}
+
 void pager_open(void) {
         int fd[2];
         const char *pager;
@@ -96,10 +109,9 @@ void pager_open(void) {
 
                 execlp("less", "less", NULL);
                 execlp("more", "more", NULL);
-                execlp("cat", "cat", NULL);
 
-                log_error("Unable to execute pager: %m");
-                _exit(EXIT_FAILURE);
+                pager_fallback();
+                /* not reached */
         }
 
         /* Return in the parent */