x11-wm/dwm: Update to 6.2

• Clean up MASTER_SITES (remove outdated websites)
• Set FREETYPEINC (now required to build) and VERSION in MARK_ARGS
• Use DISTVERSION instead of PORTVERSION for consistency
• Define a simple do-test target so that we can quickly test if the binary is at least not core dumping when started
• Improve the description of the VOLUMECONTROL option
• Replace config.mk patching with a post-configure target that makes the port respect user's CFLAGS, CPPFLAGS and LDFLAGS
• Rewrite the post-extract target using make(1) if syntax instead of sh(1) if syntax

New release announcement:
https://lists.suckless.org/dev/1902/33214.html

Reviewed by: mat
Approved by: mat (mentor)
Differential Revision: https://reviews.freebsd.org/D19416