pam: implement a zfs_key pam module

currently the pam module does:
 * load a zfs key and mounts the dataset when a session opens
 * unmounts the dataset and unloads the key when the session closes
 * when the user is logged on and changes the password, the modules
   changes the encryption key.

Signed-off-by: Felix Dörre <felix@dogcraft.de>
Closes openzfs#9886

configure.ac

@@ -94,6 +94,7 @@ AC_CONFIG_FILES([
 	contrib/initramfs/hooks/Makefile
 	contrib/initramfs/scripts/Makefile
 	contrib/initramfs/scripts/local-top/Makefile
+	contrib/pam_zfs_key/Makefile
 	contrib/pyzfs/Makefile
 	contrib/pyzfs/setup.py
 	contrib/zcp/Makefile

contrib/Makefile.am

@@ -1,2 +1,2 @@
-SUBDIRS = bash_completion.d bpftrace dracut initramfs pyzfs zcp
-DIST_SUBDIRS = bash_completion.d bpftrace dracut initramfs pyzfs zcp
+SUBDIRS = bash_completion.d bpftrace dracut initramfs pyzfs pam_zfs_key zcp
+DIST_SUBDIRS = bash_completion.d bpftrace dracut initramfs pyzfs pam_zfs_key zcp

contrib/pam_zfs_key/Makefile.am

@@ -0,0 +1,30 @@
+include $(top_srcdir)/config/Rules.am
+
+VPATH = \
+	$(top_srcdir)/module/icp \
+	$(top_srcdir)/module/zcommon \
+	$(top_srcdir)/lib/libzfs
+
+# Suppress unused but set variable warnings often due to ASSERTs
+AM_CFLAGS += $(NO_UNUSED_BUT_SET_VARIABLE)
+
+noinst_LTLIBRARIES = libpam_zfs_key.la
+
+USER_C = pam_zfs_key.c
+
+libpam_zfs_key_la_SOURCES = \
+	$(USER_C) \
+	$(KERNEL_C)
+libzfsdir = $(includedir)/libzfs
+
+libpam_zfs_key_la_LIBADD = \
+	$(top_builddir)/lib/libnvpair/libnvpair.la \
+	$(top_builddir)/lib/libuutil/libuutil.la \
+	$(top_builddir)/lib/libzfs/libzfs.la \
+	$(top_builddir)/lib/libzfs_core/libzfs_core.la
+
+libpam_zfs_key_la_LDFLAGS = -version-info 1:0:0
+
+libpam_zfs_key_la_LIBADD += -lm $(LIBSSL)
+
+EXTRA_DIST = $(USER_C)

contrib/pam_zfs_key/pam-config-zfs_key

@@ -0,0 +1,13 @@
+Name: Unlock zfs datasets for user
+Default: yes
+Priority: 128
+Auth-Type: Additional
+Auth:
+	optional	pam_zfs_key.so
+Session-Interactive-Only: yes
+Session-Type: Additional
+Session:
+	optional	pam_zfs_key.so
+Password-Type: Additional
+Password:
+	optional	pam_zfs_key.so