Polish implementation of is_same_file(). * NEWS: Announce new function. * syscalls.cc (Fcanonicalize_file_name): Add @seealso link in docstring to is_same_file(). * utils.cc (Fis_same_file): Add more extensive Programming Notes to docstring. Use iscellstr() rather than iscell() to simplify input validation. Rename variables that refer to input args to use suffix "arg" for clarity. Rename variables that refer to cell strings to "cellstr" from "cell". Use cellstr_value() to convert input octave_value to array of strings for simplified processing, rather than using cell_value() and subsequent string_value() calls on each element of cell array. Add more meaningful text to error() messages. Remove ispc() test condition for BIST test which should run correctly on any platform. Add more BIST tests for input validation.

--- a/NEWS	Sun Oct 27 17:25:36 2019 +0100
+++ b/NEWS	Sun Oct 27 10:31:33 2019 -0700
@@ -124,6 +124,7 @@
 
 ### Alphabetical list of new functions added in Octave 6
 
+* `is_same_file`
 * `lightangle`
 * `namedargs2cell`
 * `newline`