MINOR: htx: Add a function to copy a buffer in an HTX message

MINOR: htx: Add a function to copy a buffer in an HTX message

The htx_copy_msg() function can now be used to copy the HTX message stored in a
buffer in an existing HTX message. It takes care to not overwrite existing
data. If the destination message is empty, a raw copy is performed. All the
message is copied or nothing.

This function is used instead of channel_htx_copy_msg().