io: Fix incorrect alignment in allocation in libusb_alloc_transfer

After being suspicous of some code I was browsing, I tried changing
PTR_ALIGN to align to 4096 bytes instead of just to pointer size (8
bytes), then enabled ASan then ran the xusb example, and it crashed.
What ensued was a big code review of that function and related code.

- reviewed use of macros like USBI_TRANSFER_TO_LIBUSB_TRANSFER

- introduced new macros TRANSFER_PRIV_TO_USBI_TRANSFER and
USBI_TRANSFER_TO_TRANSFER_PRIV to do pointer offset conversions

- introduced some temporary variables, especially for
USBI_TRANSFER_TO_LIBUSB_TRANSFER results

- move some variable assignment and declaration together

- replaced a few uses of PTR_ALIGN to instead use the alignment macros

In particular, libusb_alloc_transfer() was not using PTR_ALIGN()
on struct usbi_transfer and could allocate a wrong size.

Closes #1418