qa: avoid hexdump skip and length options

qa: avoid hexdump skip and length options

On Debian-based distros, hexdump(1) comes from FreeBSD in bsdmainutils
package (other distros ship a Linux version from util-linux).  It turns
out that '-s skip' for anything but a regular file has been broken for
ages in FreeBSD -- instead of calling fseek(), it consumed the input
byte by byte with getchar().  This was fixed in commit 7cd5ae492557
"hexdump(1): Speed up -s flag on devices" last year, but it hasn't made
it to the Debian world yet.

bsdmainutils_9.0.6ubuntu3 in xenial had a custom patch for that, but
it's no longer there in bsdmainutils_11.1.2ubuntu1 in bionic.  hexdump
-s <large offset> is now a fancy infinite loop!

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>