Fix mount helper

Several issues related to strange mount/umount behavior were reported
and this commit should address most of them.  The original idea was
to put in place a zfs mount helper (mount.zfs).  This helper is used
to enforce 'legacy' mount behavior, and perform any extra mount argument
processing (selinux, zfsutil, etc).  This helper wasn't ready for the
0.6.0-rc1 release but with this change it's functional but needs to
extensively tested.

This change addresses the following open issues.
Closes #101
Closes #107
Closes #113
Closes #115
Closes #119

cmd/Makefile.am

@@ -1 +1,2 @@
-SUBDIRS = zfs zpool zpool_id zpool_layout zdb zinject ztest zpios zvol_id
+SUBDIRS  = zfs zpool zdb zinject ztest zpios mount_zfs
+SUBDIRS += zpool_layout zvol_id zpool_id

cmd/Makefile.in

@@ -292,7 +292,8 @@ target_vendor = @target_vendor@
 top_build_prefix = @top_build_prefix@
 top_builddir = @top_builddir@
 top_srcdir = @top_srcdir@
-SUBDIRS = zfs zpool zpool_id zpool_layout zdb zinject ztest zpios zvol_id
+SUBDIRS = zfs zpool zdb zinject ztest zpios mount_zfs zpool_layout \
+	zvol_id zpool_id
 all: all-recursive
 
 .SUFFIXES:

cmd/mount_zfs/Makefile.am

@@ -0,0 +1,28 @@
+include $(top_srcdir)/config/Rules.am
+
+DEFAULT_INCLUDES += \
+	-I$(top_srcdir)/include \
+	-I$(top_srcdir)/lib/libspl/include
+
+#
+# Ignore the prefix for the mount helper.  It must be installed in /sbin/
+# because this path is hardcoded in the mount(8) for security reasons.
+#
+sbindir=/sbin
+sbin_PROGRAMS = mount.zfs
+
+mount_zfs_SOURCES = \
+	$(top_srcdir)/cmd/mount_zfs/mount_zfs.c
+
+mount_zfs_LDADD = \
+	$(top_builddir)/lib/libspl/libspl.la \
+	$(top_builddir)/lib/libavl/libavl.la \
+	$(top_builddir)/lib/libefi/libefi.la \
+	$(top_builddir)/lib/libnvpair/libnvpair.la \
+	$(top_builddir)/lib/libunicode/libunicode.la \
+	$(top_builddir)/lib/libuutil/libuutil.la \
+	$(top_builddir)/lib/libzpool/libzpool.la \
+	$(top_builddir)/lib/libzfs/libzfs.la
+
+mount_zfs_LDFLAGS = \
+	-pthread -lm $(ZLIB) -lrt -ldl $(LIBUUID) $(LIBBLKID)