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
```