windows: Enable GC and implement bss start and end symbols

The pointers to the bss section are acquired in init.c()
by inspecting the PE header. Works for msvc and mingw.
gc: Use simple cast instead of union to silence compiler
unix: Dump default heap size in usage message.
modsocket: Fix uClibc detection.
Merge pull request #690 from stinos/msvc-gc

msvc: Enable GC