Update Files chapter in Lisp manual.

Update Files chapter in Lisp manual.

* doc/lispref/files.texi (Files): Mention magic file names as arguments.
(Reading from Files): Copyedits.
(File Attributes): Mention how to change file modes.
(Changing Files): Use standard "file permissions" terminology.
Add xref to File Attributes node.
(Locating Files): Document locate-user-emacs-file.
(Unique File Names): Recommend against using make-temp-name.

* src/buffer.c (Fget_file_buffer): Protect against invalid file
handler return value.

* src/fileio.c (Vfile_name_handler_alist): Doc fix.