trailer: rename functions to use 'trailer'

trailer: rename functions to use 'trailer'

Rename process_trailers() to interpret_trailers(), because it matches
the name for the builtin command of the same name
(git-interpret-trailers), which is the sole user of process_trailers().

In a following commit, we will move "interpret_trailers" from trailer.c
to builtin/interpret-trailers.c. That move will necessitate the growth
of the trailer.h API, forcing us to expose some additional functions in
trailer.h.

Rename relevant functions so that they include the term "trailer" in
their name, so that clients of the API will be able to easily identify
them by their "trailer" moniker, just like all the other functions
already exposed by trailer.h.

Rename `struct list_head *head` to `struct list_head *trailers` because
"head" conveys no additional information beyond the "list_head" type.

Reorder parameters for format_trailers_from_commit() to prefer

    const struct process_trailer_options *opts

as the first parameter, because these options are intimately tied to
formatting trailers. Parameters like `FILE *outfile` should be last
because they are a kind of 'out' parameter, so put such parameters at
the end. This will be the pattern going forward in this series.

Helped-by: Junio C Hamano <gitster@pobox.com>
Helped-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Linus Arver <linusa@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>