Check error returns from posix_fallocate (RHBZ#579664).

posix_fallocate has a non-standard way to return error indications.
Thus all our calls to posix_fallocate were effectively unchecked.  For
example:

  $ guestfish alloc test.img 1P
  $ echo $?
  0
  $ ll test.img
  -rw-rw-r--. 1 rjones rjones 0 2010-04-06 11:02 test.img
  $ rm test.img

With this change, errors are detected and reported properly:

  $ ./fish/guestfish alloc test.img 1P
  fallocate: File too large

This is a fix for:
https://bugzilla.redhat.com/show_bug.cgi?id=579664

daemon/fallocate.c

@@ -23,6 +23,7 @@
 #include <string.h>
 #include <unistd.h>
 #include <fcntl.h>
+#include <errno.h>
 
 #include "daemon.h"
 #include "actions.h"
@@ -41,10 +42,9 @@ do_fallocate (const char *path, int len)
   }
 
 #ifdef HAVE_POSIX_FALLOCATE
-  int r;
-
-  r = posix_fallocate (fd, 0, len);
-  if (r == -1) {
+  int err = posix_fallocate (fd, 0, len);
+  if (err != 0) {
+    errno = err;
     reply_with_perror ("%s", path);
     close (fd);
     return -1;

fish/alloc.c

@@ -24,6 +24,7 @@
 #include <unistd.h>
 #include <fcntl.h>
 #include <inttypes.h>
+#include <errno.h>
 
 #include "fish.h"
 
@@ -55,7 +56,9 @@ do_alloc (const char *cmd, int argc, char *argv[])
   }
 
 #ifdef HAVE_POSIX_FALLOCATE
-  if (posix_fallocate (fd, 0, size) == -1) {
+  int err = posix_fallocate (fd, 0, size);
+  if (err != 0) {
+    errno = err;
     perror ("fallocate");
     close (fd);
     unlink (argv[0]);