davfs2: substitute ps command

umount.davfs2 uses ps to get a process list to terminate gracefully.

On NixOS, this currently fails:

```
sh: ps: command not found
/run/current-system/sw/bin/umount.davfs:
  can't find mount.davfs-process with pid 4085;
  trying to unmount anyway.
  you propably have to remove /var/run/mount.davfs/root-x.pid manually
sh: umount: command not found
```

Fix this by patching ${procps}/bin/ps into the ps_command.

Afterwards:

```
umount.davfs: waiting for mount.davfs (pid 4106) to terminate gracefully .. OK
```

pkgs/tools/filesystems/davfs2/default.nix

@@ -1,4 +1,10 @@
-{ stdenv, fetchurl, neon, zlib }:
+{ stdenv
+, fetchurl
+, neon
+, procps
+, substituteAll
+, zlib
+}:
 
 stdenv.mkDerivation rec {
   name = "davfs2-1.5.6";
@@ -10,11 +16,21 @@ stdenv.mkDerivation rec {
 
   buildInputs = [ neon zlib ];
 
-  patches = [ ./isdir.patch ./fix-sysconfdir.patch ];
+  patches = [
+    ./isdir.patch
+    ./fix-sysconfdir.patch
+    (substituteAll {
+      src = ./0001-umount_davfs-substitute-ps-command.patch;
+      ps = "${procps}/bin/ps";
+    })
+  ];
 
   configureFlags = [ "--sysconfdir=/etc" ];
 
-  makeFlags = ["sbindir=$(out)/sbin" "ssbindir=$(out)/sbin"];
+  makeFlags = [
+    "sbindir=$(out)/sbin"
+    "ssbindir=$(out)/sbin"
+  ];
 
   meta = {
     homepage = https://savannah.nongnu.org/projects/davfs2;