xdg.hpp: Fix -Wreturn-std-move

README: Add CI and Funding badges

.builds: Add CI testing on Arch Linux

xdg.hpp: Fix style

xdg.hpp: Throw when XDG_RUNTIME_DIR has bad perms

XDG_RUNTIME_DIR directory must be 0700, throws
xdg::BaseDirectoryException otherwise.

Makefile: Improve CXXFLAGS handling and add test

xdg_test: Add test for XDG_RUNTIME_DIR

xdg_test: Improve test utils functions

Initial commit