Use inline functions to convert buffer data to scalars.

* include/host2net.h (buf16_to_ulong, buf16_to_uint): New.
(buf16_to_ushort, buf16_to_u16): New.
(buf32_to_size_t, buf32_to_ulong, buf32_to_uint, buf32_to_u32): New.

This fixes sign extension on shift problems. Hanno Böck found a case
with an invalid read due to this problem. To fix that almost all uses
of "<< 24" and "<< 8" are changed by this patch to use an inline
function from host2net.h.

(back ported from commit 2183683bd633818dd031b090b5530951de76f392)

[dkg: rebased to STABLE-BRANCH-1-4]

• Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>