Simplify port-specific manpages location after r523104

Not all of my ports are covered because:

• USES=cmake requires patching devel/cmake to undo BSD-specific defaults (A good example why BSD quirks should stay in ports, not to be upstreamed)
• USES=meson and GNU_CONFIGURE require dropping --mandir argument
• ponscripter-sekai requires Templates/BSD.local.dist to NOT create PREFIX/man