rc.subr: Eliminate about 100 forks from the boot sequence.

With the current sh, placing eval in a command substitution always results
in a fork(), even if it is the only command and only executes a single
simple command. Therefore, avoid it where it can be avoided easily.

Side effect: values starting with a hyphen and all whitespace are preserved.
The values are defaults and names for rc.conf variables and messages to be
given about obsolete ones.

MFC after:	2 weeks

etc/rc.subr

@@ -1062,7 +1062,7 @@ load_rc_config()
 
 	# Set defaults if defined.
 	for _var in $rcvar $rcvars; do
-		_defval=`eval echo "\\\$${_var}_defval"`
+		eval _defval=\$${_var}_defval
 		if [ -n "$_defval" ]; then
 			eval : \${$_var:=\$${_var}_defval}
 		fi
@@ -1070,9 +1070,9 @@ load_rc_config()
 
 	# check obsolete rc.conf variables
 	for _var in $rcvars_obsolete; do
-		_v=`eval echo \\$$_var`
-		_msg=`eval echo \\$${_var}_obsolete_msg`
-		_new=`eval echo \\$${_var}_newvar`
+		eval _v=\$$_var
+		eval _msg=\$${_var}_obsolete_msg
+		eval _new=\$${_var}_newvar
 		case $_v in
 		"")
 			;;
@@ -1765,7 +1765,7 @@ check_kern_features()
 _echoonce()
 {
 	local _var _msg _mode
-	_var=`eval echo \\$$1`
+	eval _var=\$$1
 	_msg=$2
 	_mode=$3