V2 of batch-tagging plus new dump/restore

[david at tethera.net]

this obsoletes: id:1353265498-3839-1-git-send-email-david at tethera.net

Changes based on Mark's review

- add commments for each function  in tag-util.h

- inline tag_op_list_from_string in the one place it was called 

- remove NULL terminator from tag_op_list (for mark); along with
  tag_op_list_t being opaque, I thought is was ok to leave off the
  comment on ops. But I could be convinced.

- make tag_op_list_t typedef opaque

- add BE_GENEROUS flag to parser. Currently this enables empty tags.

- handle blank lines and comments in restore and batch tagging

- fixed the commit message about "notmuch-new"

Changes based on Ethan's review:

- remove the dreaded . from first line of commit messages

- convert if on l49 of database-test.c into explicit return on error

- add comments and parens for num_tags and this_mid_len in random_corpus.c

- add check for message-id's less than length 1. I wasn't sure why we
  would disallow all message ids of length 1.

- remove declaration of parse_tag_stream

- explain "query_string += 3"

- fix typo in notmuch-dump.1