Remove pointless av_cold attributes in header files

The init functions marked as av_cold have to be executed in any case,
so there is no gain from trying to mark paths leading to such functions
as unlikely.
doc: developer: Allow tabs in the vim configuration for Automake files

While we do not use Automake in libav, this allows our config to be
used more globally without introducing unwanted breakage.
configure: Add print_3_columns helper function and use where appropriate
configure: Do not redundantly list enabled hwaccel libs

The enabled hwaccels are listed later anyway.

Signed-off-by: Mans Rullgard <mans@mansr.com>
configure: Simplify VDA header and extralibs check
configure: Simplify VDPAU header check
build: Add proper infrastructure for adding and checking host CPPFLAGS
configure: Refactor dxva2api.h dependency declarations
configure: Fix vaapi/vda/vdpau dependency declarations
ac3dec: validate channel output mode against channel count

Damaged frames can lead to a mismatch, which can cause a segfault
due to using an incorrect channel mapping.

CC:libav-stable@libav.org