Fix verilog-diff-file-with-buffer

Fix verilog-diff-file-with-buffer

* lisp/progmodes/verilog-mode.el: Fix commentary to avoid implying
XEmacs defines diff-command.
(verilog-diff-file-with-buffer): Claim compatibility with Emacs 21
rather than XEmacs in commentary, since the latter does not seem to
define diff-command.  Use get-buffer in place of likely thinko
with-temp-buffer + BUFNAME + current-buffer.  Fix unused
unwind-protect: move temporary file deletion to its unwind
forms (bug#62620).  Avoid race condition between file existence
check and deletion.  Handle list-valued diff-switches.  Avoid
passing empty argument to diff-command.