Use XDG_CONFIG_HOME for configuration file location

cwm now partially respects
https://specifications.freedesktop.org/basedir/latest/.
The default configuration file location is changed from ~/.cwmrc to
$XDG_CONFIG_HOME/cwm/cwmrc,
or, if $XDG_CONFIG_HOME is empty or unset, ~/.config/cwm/cwmrc.

cwmrc.5

@@ -14,7 +14,7 @@
 .\" ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
 .\" OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 .\"
-.Dd $Mdocdate: August 20 2025 $
+.Dd $Mdocdate: January 25 2026 $
 .Dt CWMRC 5
 .Os
 .Sh NAME
@@ -452,9 +452,17 @@ Move pointer 10 times
 .Ar moveamount
 pixels left.
 .El
+.Sh ENVIRONMENT
+.Bl -tag -width "XDG_CONFIG_HOME" -compact
+.It Ev XDG_CONFIG_HOME
+determines the default location of the configuration file.
+If unset or empty,
+.Pa ~/.config/cwm/
+is used as the value.
+.El
 .Sh FILES
-.Bl -tag -width "~/.cwmrcXXX" -compact
-.It Pa ~/.cwmrc
+.Bl -tag -width "$XDG_CONFIG_HOME/cwm/cwmrc" -compact
+.It Pa $XDG_CONFIG_HOME/cwm/cwmrc
 Default
 .Xr cwm 1
 configuration file.