physfs compilation on windows
* external calls must always be named
* physfs has to be compiled as dll
* there shouln't be external variables, implemented a function that sets buffer
* physfs extras is now integrated in main physfs
* removed physfs extras
Okay, remove previous request only if it has same parent
as this one. Fixes the last note of previous commit
(which was nearly impossible to hit, but whatever, just
cleaning implementation)