Add TaigaDisablePhysfs() and TaigaEnablePhysfs()

These functions are added solely as a workaround for PhysicsFS forcing
all files to be opened in binary mode.
Change TaigaEnablePhysfs() to TaigaResetFileInterface()

This was done because using an enable/disable system for turning
PhysicsFS on and off could have unforseen bugs if people actually
wanted it turned off the whole time, and another accidentally
enables it.