mu4e: mu4e-headers-find-if-next: move before search

when searching for the next match, go the end of the current line; when
searching for the previous match, go to the beginning of the current
line.
mu4e: org: allow customization of linking in headers mode

Depending on org-mu4e-link-query-in-headers-mode, we can link to the
query or the particular message.