Reject filenames containing NUL bytes.

Reject filenames containing NUL bytes.

Such filenames are dangerous, as Emacs would silently only use the
part up to the first NUL byte.  Reject them explicitly instead.

* src/coding.c (encode_file_name_1): New helper function.
(encode_file_name): Check that encoded filename doesn't contain a
NUL byte.
(syms_of_coding): Define 'filenamep' symbol.

* test/src/fileio-tests.el (fileio-tests/null-character): New unit
test.

* etc/NEWS: Document change.