configure: Fix silly typo in logging command of check_struct()
configure: Add sanitize_var_name convenience function

This is useful in all the places where special characters in
variable names are manually translated to underscores.
configure: Add check_builtin convenience function
configure: Use check_builtin() where appropriate
hwaccel: consistent name prefixes for start_frame/end_frame/decode_slice

Some hwaccels use name prefixes, some do not, others only use them for
some codecs.  Add prefixes everywhere for consistency.
h264: check for luma and chroma bit dept being equal

The decoder assumes a single bit depth for all the planes
while the specification allows different bit depths for luma
and chroma.

Avoid the possible problems described in CVE-2013-2277

CC: libav-stable@libav.org
vorbisdec: check memory allocations
shorten: set invalid channels count to 0

Prevent the loop shorten_decode_close from writing and freeing out of
the array boundary.

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>
shorten: K&R formatting cosmetics
shorten: report meaningful errors
shorten: use the unsigned type where needed

get_uint returns an unsigned value, use an unsigned to store
blocksize to make sure the comparison logic is correct and report
correctly the error for the channel count not supported.