This reduces the number of USB endpoints required, which frees them up
for other things.

NKRO and EXTRAKEY always use the shared endpoint.

By default, MOUSEKEY also uses it. This means it won't work as a Boot
Procotol mouse in some BIOSes, etc. If you really think your
keyboard needs to work as a mouse in your BIOS, set
MOUSE_SHARED_EP = no in your rules.mk.

By default, the core keyboard does not use the shared endpoint, as not
all BIOSes are standards compliant and that's one place you don't want
to find out your keyboard doesn't work.. If you are really confident,
you can set KEYBOARD_SHARED_EP = yes to use the shared endpoint here
too.

broke build on AVR with EXTRAKEY disabled

This avoids packets being dropped when two reports are sent in quick
succession (eg. releasing a dual role key).

No longer use wrong or unexpected endpoint IDs

V-USB has its own, understandably simple ideas about the report formats.
It already blasts the mouse and extrakeys through one endpoint with
report IDs. We just stay out of its way.

The caller(s) of host_keyboard_send expect to be able to just drop
modifiers in the mods field and not worry about whether NKRO is in use.
This is a good thing. So we just shift it over if needs be.