Honour $TMPDIR for the qemu-smb temporary directory.

diff -rc -x '*~' qemu-kvm-0.12.2-orig/net/slirp.c qemu-kvm-0.12.2/net/slirp.c
*** qemu-kvm-0.12.2-orig/net/slirp.c	2010-02-01 19:05:24.000000000 +0100
--- qemu-kvm-0.12.2/net/slirp.c	2010-02-01 19:05:54.000000000 +0100
***************
*** 476,486 ****
                       struct in_addr vserver_addr)
  {
      static int instance;
!     char smb_conf[128];
!     char smb_cmdline[128];
      FILE *f;
  
!     snprintf(s->smb_dir, sizeof(s->smb_dir), "/tmp/qemu-smb.%ld-%d",
               (long)getpid(), instance++);
      if (mkdir(s->smb_dir, 0700) < 0) {
          qemu_error("could not create samba server dir '%s'\n", s->smb_dir);
--- 476,489 ----
                       struct in_addr vserver_addr)
  {
      static int instance;
!     char smb_conf[1024];
!     char smb_cmdline[1024];
      FILE *f;
  
!     char *tmpdir = getenv("TMPDIR");
! 
!     snprintf(s->smb_dir, sizeof(s->smb_dir), "%s/qemu-smb.%ld-%d",
!              tmpdir ? tmpdir : "/tmp",
               (long)getpid(), instance++);
      if (mkdir(s->smb_dir, 0700) < 0) {
          qemu_error("could not create samba server dir '%s'\n", s->smb_dir);