Convert bsd.sdl.mk to Uses/sdl.mk

Frow now on, USES=sdl should be used to specify dependency on SDL
libraries.
USE_SDL is still required to specify components, specifying it
without USES=sdl is still supported, but deprecated, and leads to
a warning. USE_SDL=yes support was removed, and now leads to error,
USE_SDL=sdl should be used instead (all ports which still used
USE_SDL=yes were fixed). sdl.mk was simplified and modernized along
the way.

PR: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=235561
Approved by: portmgr (mat)
Exp-run by: antoine
Differential Revision: https://reviews.freebsd.org/D19075