Convert more uses of looking-at to following-char

Convert more uses of looking-at to following-char

More followup to Karl Fogel's commit a84da83c1.
* lisp/dired-aux.el (dired-add-entry, dired-subdir-hidden-p):
* lisp/dired-x.el (dired-mark-unmarked-files, dired-mark-sexp):
* lisp/help-fns.el (doc-file-to-man, doc-file-to-info):
* lisp/proced.el (proced-toggle-marks):
* lisp/progmodes/f90.el (f90-indent-line):
* lisp/ses.el (ses-load):
* lisp/tar-mode.el (tar-expunge): Replace instances of looking-at with
char comparisons using following-char.