More reliable Makefile install section

1. Use `cp --remove-destination -P -T`:
       `--remove-destination` allows installing while bspwm is running
       by unlinking files first. This means that if bspwm is currently
       using the file, it will maintain a reference to the unlinked file
       and won't get confused by the new file we're replacing it with.

       `-P` don't follow links; just copy files as-is.

       `-T` always treat the destination as a file (just in case).

2. Use `chmod`:
        Umasks aren't always predictable.

Supersedes: #161