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/davfs2/0001-umount_davfs-substitute-ps-command.patch

···
+From 0cb1321c4cbb2978318ddad73c9ee6f2a19c55c8 Mon Sep 17 00:00:00 2001
+From: Florian Klink <[e-mail]>
+Date: Sat, 11 Jan 2020 21:06:33 +0100
+Subject: [PATCH] umount_davfs: substitute ps command
+
+---
+ src/umount_davfs.c | 2 +-
+ 1 file changed, 1 insertion(+), 1 deletion(-)
+
+diff --git a/src/umount_davfs.c b/src/umount_davfs.c
+index b7019c3..a278679 100644
+--- a/src/umount_davfs.c
++++ b/src/umount_davfs.c
+@@ -157,7 +157,7 @@ main(int argc, char *argv[])
+     }
+     fclose(file);
+ 
+-    char *ps_command = ne_concat("ps -p ", pid, NULL);
++    char *ps_command = ne_concat("@ps@ -p ", pid, NULL);
+     FILE *ps_in = popen(ps_command, "r");
+     if (!ps_in) {
+         error(0, 0,
+-- 
+2.24.1
+

pkgs/tools/filesystems/davfs2/default.nix

···
-{ stdenv, fetchurl, neon, zlib }:
+{ stdenv
+, fetchurl
+, neon
+, procps
+, substituteAll
+, zlib
+}:
 
 stdenv.mkDerivation rec {
   name = "davfs2-1.5.6";
···
 
   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;