C Mode: Fix indentation of DEFUN's function's argument list.

C Mode: Fix indentation of DEFUN's function's argument list.

Formally, when DEFUNs still used knr argument lists, these were indented by
the syntactic context knr-argdecl-intro (5 columns).  Since this is no longer
the case, we now just use c-basic-offset (2 columns in "gnu" style).

* lisp/progmodes/cc-align.el (c-indent-gnu-DEFUN-intro-cont): Modernize the
indentation.  Amend the doc string accordingly.