wishlist: read-only

[chrysn]

it seems that there is currently no easy way to run pywebdav3 read-only (short of running it as a user w/o write permissions, which on linux afaict requires root privileges). please consider adding a read-only switch to the default server.

implementation-wise, i'd suggest splitting the FilesystemHandler class into a FilesystemReadHandler and a FilesystemRWHandler (apart from the last two methods, the code already looks like it's split accordingly); this would ease auditing compared to a flag set to the FilesystemHandler.