Added support for password-protected .zip files.
To use: mount a .zip file as usual, open a file as usual, but append '$' plus
the password to the end of the path, like so:
PHYSFS_File *f = PHYSFS_openRead("/path/to/my/file.txt$MyPassword");
Note that this is the "traditional" PKWARE crypto, like you might get from
PkZip 2.04g or Info-ZIP. They have more advanced cryptography in the modern
.zip spec, but that's not implemented here. If you've ever tried to unzip
an archive and were prompted for a password, you probably saw the traditional
crypto at work.
Note that this is NOT a secure thing: if you ship an app that contains a
password, someone with a debugger can retrieve it. Note also that this
password could be available to any code registered as a PHYSFS_Archiver, and
that due to how archives combine into a single file tree, the wrong archiver
can get the password, at a minimum causing it to fail because passwords don't
match.
In short: make sure you know what you're doing before you use this!
