boot device is actually read-only (even if it's using the write overlay).
* Do not create a swap file on a read-only device - this would really be a
stupid use of the write overlay (just saw this happening on an older
machine).
* Made swap_file_{add|delete}() take a const char* path - there was no reason
this was writable, and this also avoids casting away the const when adding
the default swap file.
* Minor cleanup.
git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@30975 a95241bf-73f2-0310-859d-f6bbb57e9c96
