(compilation-buffer-p): Fix braino in last change: switch to the buffer first.

(compilation-buffer-p): Fix braino in last change: switch to the buffer first.
(compilation-error-regexp-alist): Doc fix: optional cdrs give
string containing %s to produce the file name from the matched text.
(compilation-find-file): Reorder args: MARKER first, then FILENAME, DIR,
and new arg &rest FORMATS (as they appear in parsed the fileinfo lists).
Try each of the FORMATS in each directory tried.
(compilation-next-error-locus): Apply compilation-find-file to the FILEINFO
list.
(compilation-parse-errors): Instead of a cons (DIR . FILE), make a list
(FILE DIR [FORMATS...]) using the 4th cdr of the matching elt of
regexp-alist.