Add support for btrfs subvolumes for user homes

new switch added to useradd command, --btrfs-subvolume-home. When
specified *and* the filesystem is detected as btrfs, it will create a
subvolume for user's home instead of a plain directory. This is done via
`btrfs subvolume` command.  Specifying the new switch while trying to
create home on non-btrfs will result in an error.

userdel -r will handle and remove this subvolume transparently via
`btrfs subvolume` command. Previosuly this failed as you can't rmdir a
subvolume.

usermod, when moving user's home across devices, will detect if the home
is a subvolume and issue an error messages instead of copying it. Moving
user's home (as subvolume) on same btrfs works transparently.

libmisc/btrfs.c

@@ -0,0 +1,94 @@
+#include <linux/btrfs_tree.h>
+#include <linux/magic.h>
+#include <sys/statfs.h>
+
+#include "prototypes.h"
+
+
+static int run_btrfs_subvolume_cmd(const char *subcmd, const char *arg1, const char *arg2)
+{
+	int status = 0;
+	const char *cmd = "/sbin/btrfs";
+	const char *argv[] = {
+		strrchr(cmd, '/'),
+		"subvolume",
+		subcmd,
+		arg1,
+		arg2,
+		NULL
+	};
+
+	if (argv[0] == NULL)
+		argv[0] = cmd;
+	else
+		argv[0] = argv[0] + 1;
+
+	if (access(cmd, X_OK)) {
+		return 1;
+	}
+
+	if (run_command(cmd, argv, NULL, &status))
+		return -1;
+	return status;
+}
+
+
+int btrfs_create_subvolume(const char *path)
+{
+	return run_btrfs_subvolume_cmd("create", path, NULL);
+}
+
+
+int btrfs_remove_subvolume(const char *path)
+{
+	return run_btrfs_subvolume_cmd("delete", "-C", path);
+}
+
+
+/* Adapted from btrfsprogs */
+/*
+ * This intentionally duplicates btrfs_util_is_subvolume_fd() instead of opening
+ * a file descriptor and calling it, because fstat() and fstatfs() don't accept
+ * file descriptors opened with O_PATH on old kernels (before v3.6 and before
+ * v3.12, respectively), but stat() and statfs() can be called on a path that
+ * the user doesn't have read or write permissions to.
+ *
+ * returns:
+ *   1 - btrfs subvolume
+ *   0 - not btrfs subvolume
+ *  -1 - error
+ */
+int btrfs_is_subvolume(const char *path)
+{
+	struct stat st;
+	int ret;
+
+	ret = is_btrfs(path);
+	if (ret <= 0)
+		return ret;
+
+	ret = stat(path, &st);
+	if (ret == -1)
+		return -1;
+
+	if (st.st_ino != BTRFS_FIRST_FREE_OBJECTID || !S_ISDIR(st.st_mode)) {
+		return 0;
+	}
+
+	return 1;
+}
+
+
+/* Adapted from btrfsprogs */
+int is_btrfs(const char *path)
+{
+	struct statfs sfs;
+	int ret;
+
+	ret = statfs(path, &sfs);
+	if (ret == -1)
+		return -1;
+
+	return sfs.f_type == BTRFS_SUPER_MAGIC;
+}
+