zpool: Add slot power control, print power status

Add `zpool` flags to control the slot power to drives.  This assumes
your SAS or NVMe enclosure supports slot power control via sysfs.

The new `--power` flag is added to `zpool offline|online|clear`:

    zpool offline --power <pool> <device>    Turn off device slot power
    zpool online --power <pool> <device>     Turn on device slot power
    zpool clear --power <pool> [device]      Turn on device slot power

If the ZPOOL_AUTO_POWER_ON_SLOT env var is set, then the '--power'
option is automatically implied for `zpool online` and `zpool clear`
and does not need to be passed.

zpool status also gets a --power option to print the slot power status.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Mart Frauenlob <AllKind@fastest.cc>
Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Closes #15662

lib/libzutil/zutil_pool.c

@@ -28,6 +28,7 @@
 #include <string.h>
 #include <sys/nvpair.h>
 #include <sys/fs/zfs.h>
+#include <math.h>
 
 #include <libzutil.h>
 
@@ -144,3 +145,33 @@ zpool_history_unpack(char *buf, uint64_t bytes_read, uint64_t *leftover,
 	*leftover = bytes_read;
 	return (0);
 }
+
+/*
+ * Floating point sleep().  Allows you to pass in a floating point value for
+ * seconds.
+ */
+void
+fsleep(float sec)
+{
+	struct timespec req;
+	req.tv_sec = floor(sec);
+	req.tv_nsec = (sec - (float)req.tv_sec) * NANOSEC;
+	nanosleep(&req, NULL);
+}
+
+/*
+ * Get environment variable 'env' and return it as an integer.
+ * If 'env' is not set, then return 'default_val' instead.
+ */
+int
+zpool_getenv_int(const char *env, int default_val)
+{
+	char *str;
+	int val;
+	str = getenv(env);
+	if ((str == NULL) || sscanf(str, "%d", &val) != 1 ||
+	    val < 0) {
+		val = default_val;
+	}
+	return (val);
+}