devel/allegro: add sndio support, cleanup port

[tobik]

Summary

Commit 1:

- Add sndio support [1]
- Remove ALSA from default options
- Switch to options helpers
- Move audio output options to their own option group
- Make OSS support an explicit option, so that it can be turned off
- Remove OGG option and make it mandatory: audio/libogg is already a
  dependency via audio/libvorbis
- Make the port simpler by not building the examples and not
  installing the example games (if necessary installing them can be done
  later in a separate port)
- Add LICENSE
- Fix WWW
- Allegro never directly links with libpng or libGLU; make them a build
  dependency only
- Reset maintainer 

PR:		215838
Obtained from:	OpenBSD [1]
Approved by:	??? (mentor), maintainer timeout (5 months)
Differential Revision:	https://reviews.freebsd.org/D?????

Commit 2:

Add explicit libpng dependency to fix the build of games/opensonic
after the recent devel/allegro update.

PR:		215838
Approved by:	??? (mentor), [e-mail] (maintainer)
Differential Revision:	https://reviews.freebsd.org/D?????

Test Plan

poudriere testport 11.0/amd64 ok

+ test builds of dependent (and non-broken) ports

audio/dumb-allegro
devel/adime
devel/fifechan
emulators/o2em
games/ags
games/alex4
games/atanks
games/fargoal
games/garden-of-coloured-lights
games/liquidwar
games/opensonic
games/paintown
graphics/alpng
graphics/aseprite
math/chryzodus