For +=/-=, add . and / to convenience characters

sysrc(8) supports key+=value and key-=value, but can be told what the
delimiter is by being passed as char1 (e.g., "sysrc key+=",value" to use a
comma as the delimiter instead of space). For convenience, if the first char
is alpha-numeric, it is assumed you wanted whitespace as the delimiter.

However, if you naively (as I just did) execute:
	sysrc rc_conf_files+=/etc/rc.conf.other
the result is unexpected.

This commit makes `.' and `/' in-addition to alpha-numeric first-characters
to cause the default of whitespace to be used as the delimiter. This also
means that you can no longer use these as a delimiter.

usr.sbin/sysrc/sysrc

@@ -790,7 +790,7 @@ while [ $# -gt 0 ]; do
 			delim="${add%"${add#?}"}" # first character
 			oldIFS="$IFS"
 			case "$delim" in
-			""|[$IFS]|[a-zA-Z0-9]) delim=" " ;;
+			""|[$IFS]|[a-zA-Z0-9./]) delim=" " ;;
 			*) IFS="$delim"
 			esac
 			new="$before"
@@ -812,7 +812,7 @@ while [ $# -gt 0 ]; do
 			delim="${remove%"${remove#?}"}" # first character
 			oldIFS="$IFS"
 			case "$delim" in
-			""|[$IFS]|[a-zA-Z0-9]) delim=" " ;;
+			""|[$IFS]|[a-zA-Z0-9./]) delim=" " ;;
 			*) IFS="$delim"
 			esac
 			new=

usr.sbin/sysrc/sysrc.8

@@ -275,6 +275,10 @@ it is added
 .Pp
 For convenience, if the first character is alpha-numeric
 .Pq letters A-Z, a-z, or numbers 0-9 ,
+dot
+.Pq Li . ,
+or slash
+.Pq Li / ,
 .Nm
 uses the default setting of whitespace as separator.
 For example, the above and below statements are equivalent since
@@ -329,6 +333,10 @@ it is removed
 .Pp
 For convenience, if the first character is alpha-numeric
 .Pq letters A-Z, a-z, or numbers 0-9 ,
+dot
+.Pq Li . ,
+or slash
+.Pq Li / ,
 .Nm
 uses the default setting of whitespace as separator.
 For example, the above and below statements are equivalent since