Make WARNS=6 the default for libexec/.

Just like bin/ and sbin/, I think setting WARNS to the highest value
possible will make it more attractive for people to fix warnings.

- The WARNS variable is set in the Makefile in the directory of the
  application itself, making it more likely that it will be removed out
  of curiosity to see what happens.
- New applications will most likely build with WARNS=6 out of the box,
  because the author would more likely fix the warnings during
  development than lower WARNS.

Unfortunately almost all apps in libexec require a lowered value of
WARNS.

libexec/bootpd/Makefile

@@ -5,6 +5,8 @@ PROG=	bootpd
 CFLAGS+= -DETC_ETHERS
 CFLAGS+= -DSYSLOG -DDEBUG -DVEND_CMU
 
+WARNS?=	2
+
 SUBDIR= bootpgw tools
 
 SRCS=	bootpd.c dovend.c readfile.c hash.c dumptab.c \

libexec/bootpd/Makefile.inc

@@ -1,3 +1,5 @@
 # $FreeBSD$
 
 BINDIR?=	/usr/libexec
+
+WARNS?=		1

libexec/bootpd/tools/Makefile.inc

@@ -2,3 +2,5 @@
 # $FreeBSD$
 
 BINDIR=		/usr/sbin
+
+WARNS?=		1