Read the /etc/hostid file directly.

Deprecate the /usr/bin/hostid call by reading the /etc/hostid file
directly. Add the spl_hostid_path parameter to override the default
/etc/hostid path.

Rename the set_hostid() function to hostid_exec() to better reflect
actual behavior and complement the new hostid_read() function.

Use HW_INVALID_HOSTID as the spl_hostid sentinel value because
zero seems to be a valid gethostid() result on Linux.

include/sys/systeminfo.h

@@ -30,4 +30,8 @@
 						/* to hold a decimal or hex */
 						/* hostid string */
 
+/* Supplemental definitions for Linux. */
+#define HW_HOSTID_PATH		"/etc/hostid"   /* binary configuration file */
+#define HW_HOSTID_MASK		0xFFFFFFFF 	/* significant hostid bits */
+
 #endif /* SPL_SYSTEMINFO_H */