fmt_scan(): Avoid undefined behaviour from overlapping strncpy(3).

fmt_scan(): Avoid undefined behaviour from overlapping strncpy(3).

M. Levinson pointed out in private email that pcc(1) tickles a bug that
causes undefined behaviour due to part of `buffer[]', accessed by `str',
being strncpy(3) back to its start.  Fix by using memmove(3) to slide
`str' back to `buffer's beginning whenever it may have wandered off.

Fixes e8635a8a that added `%(unquote)', but using `%(trim)' twice was
also faulty in the same way and pre-dates git.