cperl-mode.el: Make sure cperl-file-style is set buffer-local

cperl-mode.el: Make sure cperl-file-style is set buffer-local

* lisp/progmodes/cperl-mode.el (cperl-file-style): Add description
what the options actually do.
(cperl-menu): Split the menu entry "Indent styles" into "Default
indent styles" and "Indent styles for current buffer"
(cperl--set-file-style): call `cperl-file-style' instead of
`cperl-set-style'.  This completes the fix for Bug#17948.
(cperl-set-style): Explain when to use `cperl-file-style'.
Use `set-default-toplevel-value' instead of `set'.
(cperl-set-style-back): Use `set-default-toplevel-value' instead
of `set'.
(cperl-file-style): New command to set the file style for the
current buffer.

* etc/NEWS: Announce the new command cperl-file-style.