physfs: make PHYSFS_close behave like fclose with respect to error codes

Most of the PHYSFS_* functions behave like their fopen/fclose/etc counterparts,
returning 0 on success and nonzero on failure. PHYSFS_close is a special
snowflake and returns zero on failure.

This fixes this behavior so that functions expecting PHYSFS_close to behave
like fclose don't freak out.

Signed-off-by: Steven Noonan <steven@uplinklabs.net>