Add sndio backend

While here

• Add LICENSE_FILE, USES=localbase
• Strip library
• Group audio backend options
• Use options helpers for VGL

I think the configure script patch needs to be explained. OpenBSD
regenerates the configure script from configure.in after patching
sndio detection into it. However SDL 1.2 requires an older autoconf
version (2.59) which is not available on FreeBSD. Lots of errors when
using autoconf 2.69.

Since I was unable to regenerate it I opted to edit configure
manually. So it does the bare minimum to wire the backend sources
into the build and make sure that SNDIO_CONFIGURE_ENABLE can do its
job in enabling/disabling sndio support.

Approved by: lme (mentor)
Obtained from: OpenBSD
Differential Revision: https://reviews.freebsd.org/D10658