Simplify hostid logic

There is plenty of compatibility code for a hw_hostid
that isn't used by anything. At the same time, there are apparently
issues with the current hostid logic. coredumb in #zfsonlinux on
freenode reported that Fedora 17 changes its hostid on every boot, which
required force importing his pool. A suggestion by wca was to adopt
FreeBSD's behavior, where it treats hostid as zero if /etc/hostid does
not exist

Adopting FreeBSD's behavior permits us to eliminate plenty of code,
including a userland helper that invokes the system's hostid as a
fallback.

Signed-off-by: Richard Yao <ryao@cs.stonybrook.edu>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #224

include/sys/sysmacros.h

@@ -143,7 +143,6 @@
 /* Missing globals */
 extern char spl_version[32];
 extern unsigned long spl_hostid;
-extern char hw_serial[11];
 
 /* Missing misc functions */
 extern int highbit(unsigned long i);

include/sys/systeminfo.h

@@ -25,7 +25,6 @@
 #ifndef _SPL_SYSTEMINFO_H
 #define _SPL_SYSTEMINFO_H
 
-#define HW_INVALID_HOSTID	0xFFFFFFFF	/* an invalid hostid */
 #define HW_HOSTID_LEN		11		/* minimum buffer size needed */
 						/* to hold a decimal or hex */
 						/* hostid string */