lib: Start all function names in notmuch-private.h with

lib: Start all function names in notmuch-private.h with

As noted in devel/STYLE, every private library function should start
with _notmuch. This patch corrects function naming that did not adhere
to this style in lib/notmuch-private.h. In particular, the old function
names that now begin with _notmuch are

    notmuch_sha1_of_file
    notmuch_sha1_of_string
    notmuch_message_file_close
    notmuch_message_file_get_header
    notmuch_message_file_open
    notmuch_message_get_author
    notmuch_message_set_author

Signed-off-by: Charles Celerier <cceleri@cs.stanford.edu>