string-util: add a mechanism for strextend_with_separator() for specifying "ignore...

string-util: add a mechanism for strextend_with_separator() for specifying "ignore" arguments

in strv_new() we have STRV_IGNORE for skipping over an argument in the
argument list. Let's add the same to strextend_with_separator():

strextend_with_separator(&x, "foo", POINTER_MAX, "bar");

will result in "foobar" being appended to "x". (POINTER_MAX Which is
different from NULL, which terminates the argument list).

This is useful for ternary op situations.

(We should probably get rid of STRV_IGNORE and just use POINTER_MAX
everywhere directly, but that's for another time.)