contrib: pam_mount: add documentation and wrapper

See ticket #34

contrib/pam_mount/README.md

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+Mounting gocryptfs on login using pam_mount
+===========================================
+
+This works on Fedora 24 with active SELinux. Feedback on other platforms
+is welcome.
+
+gocryptfs
+---------
+
+Copy the `gocryptfs` binary and `gocryptfs_pam_mount.bash` into
+`/usr/local/bin` .
+
+The bash wrapper is neccessary because of the different calling
+conventions between pam_mount and gocryptfs.
+
+Create a gocryptfs filesystem:
+```
+$ mkdir /home/testuser/cipher /home/testuser/plain
+$ gocryptfs -init /home/testuser/cipher
+```
+
+pam_mount config
+----------------
+
+Put the following into `/etc/security/pam_mount.conf.xml`, just before
+the closing `</pam_mount>` tag at the bottom:
+
+```
+<volume user="testuser" fstype="fuse" options="defaults"
+path="/usr/local/bin/gocryptfs_pam_mount.bash#/home/%(USER)/cipher"
+mountpoint="/home/%(USER)/plain" />
+```
+
+If you want to disable the display of the masterkey on mount, replace
+`options="defaults"` with `options="quiet"`.
+
+PAM config
+----------
+
+An example `/etc/pam.d/login` on Fedora 24 is shown below. pam_mount
+MUST be called AFTER `pam_selinux.so open` because that puts us in the
+right SELinux context.
+
+```
+#%PAM-1.0
+auth       substack     system-auth
+auth       include      postlogin
+account    required     pam_nologin.so
+account    include      system-auth
+password   include      system-auth
+session    required     pam_selinux.so close
+session    required     pam_loginuid.so
+session    optional     pam_console.so
+session    required     pam_selinux.so open
+session    required     pam_namespace.so
+# vvv insert pam_mount here
+session optional pam_mount.so
+# ^^^ insert pam_mount here
+session    optional     pam_keyinit.so force revoke
+session    include      system-auth
+session    include      postlogin
+-session   optional     pam_ck_connector.so
+```

contrib/pam_mount/gocryptfs_pam_mount.bash

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+#!/bin/bash
+#
+# Simple bash script to transform the command-line arguments that
+# pam_mount passes to gocryptfs into something that gocryptfs
+# understands.
+#
+# Currently understood: nonempty,allow_other,quiet.
+# Unknown options are ignored.
+
+exec >&2
+set -eu
+
+if [[ $# != 4 ]]; then
+	MYNAME=$(basename $0)
+	echo "$MYNAME: expected 4 arguments, got $#"
+	echo "Example: $MYNAME /home/user.crypt /home/user.plain -o allow_other"
+	echo "Example: $MYNAME /home/user.crypt /home/user.plain -o defaults"
+	exit 1
+fi
+
+SRC=$1
+DST=$2
+GOPTS=""
+for OPT in nonempty allow_other quiet; do
+	if [[ $4 == *$OPT* ]]; then
+		GOPTS="$GOPTS -$OPT"
+	fi
+done
+
+cd "$(dirname "$0")"
+exec ./gocryptfs $GOPTS $SRC $DST