Enable visiting FIFOs as files

Enable visiting FIFOs as files

* doc/lispref/files.texi (Reading from Files): Document new
`if-regular' value of REPLACE.

* etc/NEWS: Announce the new value.

* lisp/files.el (basic-save-buffer-2): Demote errors saving
backup files, since FIFOs cannot be copied.
(revert-buffer-insert-file-contents--default-function): Supply
`if-regular' instead of t as REPLACE.

* src/fileio.c (selinux_enabled_p): New function.
(Fcopy_file, Ffile_selinux_context, Fset_file_selinux_context):
Call that function to ascertain if SELinux applies to a file.
(read_non_regular): Don't assume `emacs_fd_read' always returns
int.
(Finsert_file_contents): If REPLACE is if-regular and FILENAME
is a special non-seekable file, fall back to erasing the buffer
before inserting the contents of that file.
(syms_of_fileio) <Qif_regular>: New symbol.