Use inline functions to convert buffer data to scalars.

* common/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.

Commit 91b826a38880fd8a989318585eb502582636ddd8 was not enough to
avoid all sign extension on shift problems. Hanno Böck found a case
with an invalid read due to this problem. To fix that once and for
all almost all uses of "<< 24" and "<< 8" are changed by this patch to
use an inline function from host2net.h.

• Signed-off-by: Werner Koch <wk@gnupg.org>