trailer: spread usage of "trailer_block" language

trailer: spread usage of "trailer_block" language

Deprecate the "trailer_info" struct name and replace it with
"trailer_block". This is more readable, for two reasons:

  1. "trailer_info" on the surface sounds like it's about a single
     trailer when in reality it is a collection of one or more trailers,
     and

  2. the "*_block" suffix is more informative than "*_info", because it
     describes a block (or region) of contiguous text which has trailers
     in it, which has been parsed into the trailer_block structure.

Rename the

    size_t trailer_block_start, trailer_block_end;

members of trailer_info to just "start" and "end". Rename the "info"
pointer to "trailer_block" because it is more descriptive. Update
comments accordingly.

Signed-off-by: Linus Arver <linus@ucla.edu>
Signed-off-by: Taylor Blau <me@ttaylorr.com>