Implement per-buffer parameters

Some parameters, line readonly and format, would be better specified per buffer instead of being a single parameter that applies to all buffers.

For example if you set a file read-only, all open files become read-only.
Similarly, if you merely insert an msdos file into another with autodetect on,
all the buffers will then be written in msdos mode.

Interestingly, there are already per-buffer flags, they are just not attached to the parameters mechanism.
