This attribute is a generalized version of cryptHomeLuks for creating an
entry in /etc/security/pam_mount.conf.xml. It lets the configuration
control all the attributes of the <volume> entry, instead of just the
path. The default path remains the value of cryptHomeLuks, for
compatibility.
Change order of pam_mount.conf.xml so that users can override the preset configs.
My use case is to mount a gocryptfs (a fuse program) volume. I can not do that in current order.
Because even if I change the `<fusermount>` and `<fuserumount>` by add below to extraVolumes
```
<fusemount>${pkgs.fuse}/bin/mount.fuse %(VOLUME) %(MNTPT) "%(before=\"-o \" OPTIONS)"</fusemount>
<fuseumount>${pkgs.fuse}/bin/fusermount -u %(MNTPT)</fuseumount>
```
mount.fuse still does not work because it can not find `fusermount`. pam_mount will told stat /bin/fusermount failed.
Fine, I can add a `<path>` section to extraVolumes
```
<path>${pkgs.fuse}/bin:${pkgs.coreutils}/bin:${pkgs.utillinux}/bin</path>
```
but then the `<path>` section is overridden by the hardcoded `<path>${pkgs.utillinux}/bin</path>` below. So it still does not work.
