Breaking changes: `query_default_app()` now returns the 'Exec=' value of
the desktop file instead of just the binary name.

The current way was missing very common usecases, like passing arguments
to the program, or giving it environment variables, and it didn’t even
give a valid path when the full path wasn’t present.

The Exec line must be a POSIX shell command.

It now ignores matches from the wrong section, and should be negligibly
slower, while still introducing no dependency.

These directories haven’t existed in some decades, there is no need to
support them in a new implementation of xdg-utils in 2020. :)

/bin/sh might be a symbolic link