etcupdate: Make nobuild the default

The common case for etcupdate is to run it after building and installing
the world, in which case we already have an object directory to draw on.
Add a -b option to turn nobuild off (opposite of -B), and turn nobuild
on by default.

MFC after:	1 week
Reviewed by:	jhb, emaste
Differential Revision:	https://reviews.freebsd.org/D57306

usr.sbin/etcupdate/etcupdate.8

@@ -23,7 +23,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd April 30, 2026
+.Dd May 27, 2026
 .Dt ETCUPDATE 8
 .Os
 .Sh NAME
@@ -31,7 +31,8 @@
 .Nd "manage updates to system files not updated by installworld"
 .Sh SYNOPSIS
 .Nm
-.Op Fl npBFN
+.Op Fl npFN
+.Op Fl B | Fl b
 .Op Fl d Ar workdir
 .Op Fl r | Fl s Ar source | Fl t Ar tarball
 .Op Fl A Ar patterns
@@ -42,7 +43,8 @@
 .Op Fl m Ar make
 .Nm
 .Cm build
-.Op Fl BN
+.Op Fl N
+.Op Fl B | Fl b
 .Op Fl d Ar workdir
 .Op Fl s Ar source
 .Op Fl L Ar logfile
@@ -58,7 +60,8 @@
 .Op Fl L Ar logfile
 .Nm
 .Cm extract
-.Op Fl BN
+.Op Fl N
+.Op Fl B | Fl b
 .Op Fl d Ar workdir
 .Op Fl s Ar source | Fl t Ar tarball
 .Op Fl D Ar destdir
@@ -388,15 +391,16 @@ variable or the
 option will not be installed.
 .It Fl B
 Do not build generated files in a private object tree.
-Instead,
+Instead, reuse the generated files from a previously built object tree
-reuse the generated files from a previously built object tree that matches
+that matches the source tree.
-the source tree.
+This is the default.
-This can be useful to avoid gratuitous conflicts in
+.It Fl b
-.Xr sendmail 8
+Build generated files in a private object tree before trying to
-configuration
+compare anything.
-files when bootstrapping.
+This is necessary in the uncommon case where
-It can also be useful for building a tarball that matches a specific
+.Nm
-world build.
+is not run after a
+.Cm buildworld .
 .It Fl D Ar destdir
 Specify an alternate destination directory as the target of a merge.
 This is analogous to the

usr.sbin/etcupdate/etcupdate.sh

@@ -1780,12 +1780,15 @@ rerun=
 always=
 dryrun=
 ignore=
-nobuild=
+nobuild=YES
 preworld=
 noroot=
 difflistonly=
-while getopts "d:lm:nprs:t:A:BD:FI:L:M:N" option; do
+while getopts "bd:lm:nprs:t:A:BD:FI:L:M:N" option; do
 	case "$option" in
+		b)
+			nobuild=
+			;;
 		d)
 			WORKDIR=$OPTARG
 			;;