Drastically improve coop sync

Service the network while wiping the screen; avoids huge command backlog
which causes sync problems down the road

Serialize basetic separately, rather than as a delta between it and
gametic

Serialize numspechit; probably not necessary but for the sake of
completeness...

Break i_sound into i_sound, i_music and i_emusic

Pull the checks for snd_card, nosfxparm, mus_card and nomusicparm down
into i_sound/music/emusic; avoids RNG sync problems

Use a different RNG for sound randomness; avoids RNG sync problems

Make N_GetMessageRecipientBuffer static

Add HAVE_MIXER to config.h and CMakeLists.txt; this really ought to be
consolidated with HAVE_LIBSDL_MIXER though

Update style guide with some guidance on blocks after
return/continue/etc.

Reformat random number table in m_random.c; due to the bad initial
re-indentation, the alignment got roasted; fixed

Reformatting