[physfs] request for 'openReadWrite'

[Ryan C. Gordon]

> Is this addition compatible with the scope of the aforementioned library?

Is the problem that there's no read+write call, or is the problem that 
writing options are either truncate or append?

Is it not reasonable to open the same file twice, once for read and once 
for write? Append mode will let you seek back, despite the name.

I'd be hesitant to add an openReadWrite() call; it brings up all sorts 
of questions about which file you should be actually be accessing, and 
I'd rather dodge that question entirely.

Also, out of curiousity: what are you doing that requires you to read 
and write the same file at the same time? I'm sure there are legitimate 
use cases for that, I'm just curious.

--ryan.