Move cgo portions of FreeBSD devstat collector into own file

Embedding 100 lines of code in a comment doesn't make for good reading,
editing or code quality.

collector/devstat_freebsd.h

@@ -0,0 +1,41 @@
+#include <devstat.h>
+#include <fcntl.h>
+#include <libgeom.h>
+#include <limits.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+typedef struct {
+	uint64_t	read;
+	uint64_t	write;
+	uint64_t	free;
+} Bytes;
+
+typedef struct {
+	uint64_t	other;
+	uint64_t	read;
+	uint64_t	write;
+	uint64_t	free;
+} Transfers;
+
+typedef struct {
+	double		other;
+	double		read;
+	double		write;
+	double		free;
+} Duration;
+
+typedef struct {
+	char		device[DEVSTAT_NAME_LEN];
+	int		unit;
+	Bytes		bytes;
+	Transfers	transfers;
+	Duration	duration;
+	long		busyTime;
+	uint64_t	blocks;
+} Stats;
+
+
+int _get_ndevs();
+Stats _get_stats(int i);