m4: avoid spurious output with unrecognized format specifiers.
* modules/format.c (format): Advance the loop invariants past an
unrecognized format specifier to avoid printing the specifier
character on the next pass.
* doc/m4.texi (Format): Don't expect unrecognized specifiers to
be output by format macro.
* tests/null.out: Don't expect unrecognized null specifier to be
output.
* tests/generate.awk (new_test): Treat `ignore' in expected error
output the same as Autotest.
* doc/m4.texi (Format): Use special `ignore' string rather than
unsupported @comment xerr magic.
Signed-off-by: Gary V. Vaughan <gary@gnu.org>