Tweak documentation to RB_ constants to reflect current use

RB_ASKNAME is no longer instructions to the boot loader to request a
prompt for which kernel to boot. Instead, it asks for what the root
file system to use. RB_INITNAME is unused, and never has been in
FreeBSD as far as I can tell. Remove it from the documentation and fix
comment. RB_SELFTEST and RB_MINIROOT likewise (though they were
completely undocumented). These last three constants can likely just
be deleted as nothing references them (even to set useless bits).

RB_ASKNAME doesn't actually survive reboot, however, so needs to be
communicated to the bootloader via other means. If the bootloader sets
it, though, it will be honored.

lib/libc/sys/reboot.2

@@ -28,7 +28,7 @@
 .\"     @(#)reboot.2	8.1 (Berkeley) 6/4/93
 .\" $FreeBSD$
 .\"
-.Dd October 24, 2017
+.Dd July 10, 2018
 .Dt REBOOT 2
 .Os
 .Sh NAME
@@ -58,18 +58,13 @@ options, defined in the include file
 .In sys/reboot.h ,
 to be passed
 to the new kernel or the new bootstrap and init programs.
-.Bl -tag -width RB_INITNAMEA
+.Bl -tag -width RB_POWERCYCLE
 .It Dv RB_AUTOBOOT
 The default, causing the system to reboot in its usual fashion.
 .It Dv RB_ASKNAME
-Interpreted by the bootstrap program itself, causing it to
-prompt on the console as to what file should be booted.
-Normally, the system is booted from the file
-.Dq Ar xx Ns No (0,0)kernel ,
-where
-.Ar xx
-is the default disk name,
-without prompting for the file name.
+Normally the system only prompts the user if the loader specified root
+file system has an error.
+This flag forces it to always prompt the user for the root partition.
 .It Dv RB_DFLTROOT
 Use the compiled in root device.
 Normally, the system uses the device from which it was booted
@@ -96,13 +91,6 @@ driver implements this feature.
 After halting, the shutdown code will do what it can to turn
 off the power.
 This requires hardware support.
-.It Dv RB_INITNAME
-An option allowing the specification of an init program (see
-.Xr init 8 )
-other than
-.Pa /sbin/init
-to be run when the system reboots.
-This switch is not currently available.
 .It Dv RB_KDB
 Load the symbol table and enable a built-in debugger in the system.
 This option will have no useful function if the kernel is not configured

sys/sys/reboot.h

@@ -41,23 +41,23 @@
  */
 #define	RB_AUTOBOOT	0	/* flags for system auto-booting itself */
 
-#define	RB_ASKNAME	0x001	/* ask for file name to reboot from */
+#define	RB_ASKNAME	0x001	/* force prompt of device of root filesystem */
 #define	RB_SINGLE	0x002	/* reboot to single user only */
 #define	RB_NOSYNC	0x004	/* dont sync before reboot */
 #define	RB_HALT		0x008	/* don't reboot, just halt */
-#define	RB_INITNAME	0x010	/* name given for /etc/init (unused) */
+#define	RB_INITNAME	0x010	/* Unused placeholder to specify init path */
 #define	RB_DFLTROOT	0x020	/* use compiled-in rootdev */
 #define	RB_KDB		0x040	/* give control to kernel debugger */
 #define	RB_RDONLY	0x080	/* mount root fs read-only */
 #define	RB_DUMP		0x100	/* dump kernel memory before reboot */
-#define	RB_MINIROOT	0x200	/* mini-root present in memory at boot time */
+#define	RB_MINIROOT	0x200	/* Unused placeholder */
 #define	RB_VERBOSE	0x800	/* print all potentially useful info */
 #define	RB_SERIAL	0x1000	/* use serial port as console */
 #define	RB_CDROM	0x2000	/* use cdrom as root */
 #define	RB_POWEROFF	0x4000	/* turn the power off if possible */
 #define	RB_GDB		0x8000	/* use GDB remote debugger instead of DDB */
 #define	RB_MUTE		0x10000	/* start up with the console muted */
-#define	RB_SELFTEST	0x20000	/* don't complete the boot; do selftest */
+#define	RB_SELFTEST	0x20000	/* unused placeholder */
 #define	RB_RESERVED1	0x40000	/* reserved for internal use of boot blocks */
 #define	RB_RESERVED2	0x80000	/* reserved for internal use of boot blocks */
 #define	RB_PAUSE	0x100000 /* pause after each output line during probe */