Make cmd_parse_fetch() more precise about setting reopen/check flags.

Make cmd_parse_fetch() more precise about setting reopen/check flags.

Previously any FETCH with FLAGS would result in either
  idata->reopen |= IMAP_EXPUNGE_PENDING;
  -or-
  idata->check_status = IMAP_FLAGS_PENDING;
being set.

This is unnecessary in the case of responses to FLAGS.SILENT updates
sent by mutt (which seem to commonly happen now-a-days).

Change imap_set_flags() to compare the old server flags against the
new ones, and report when _those_ updates would/did result in a local
header flag change.  Only set one of the reopen/check_status flags in
the event of an actual change (or potential change if a local change
has been made to the header.)