Don't emulate dd's skip=N for *all* character devices; lseek works

Don't emulate dd's skip=N for *all* character devices;  lseek works
just fine on non-tape character devices like /dev/mem and /dev/kmem.

(buggy_lseek_support) [__linux__]: Emulate `skip=N'
behavior using reads, not lseek for Linux tape devices (major == 9).
Kernel lseek support for tapes is broken, up to and including
linux-2.4.16.  Reported by Herbert Xu.