libarchive: Force GNU iconv compatibility on FreeBSD

When libarchive is compiled with FreeBSD's native iconv instead of
libiconv, as happens with libarchive in the base system, we need to
configure iconv(3) to handle invalid sequences by returning -1, as
iconv_strncat_in_locale() assumes GNU iconv semantics.

This corresponds to upstream PR 3056.

PR:		294577
MFC after:	1 week

contrib/libarchive/libarchive/archive_string.c

@@ -1314,7 +1314,17 @@ create_sconv_object(const char *fc, const char *tc,
 			else if (strcmp(fc, "CP932") == 0)
 				sc->cd = iconv_open(tc, "SJIS");
 		}
-#if defined(_WIN32) && !defined(__CYGWIN__)
+#if defined(__FreeBSD__) && !defined(HAVE_LIBICONV)
+		/*
+		 * FreeBSD's native iconv() by default returns the number of
+		 * invalid characters in the input string, as specified by
+		 * POSIX, but iconv_strncat_in_locale() assumes GNU iconv
+		 * semantics.
+		 */
+		int v = 1;
+
+		(void)iconvctl(sc->cd, ICONV_SET_ILSEQ_INVALID, &v);
+#elif defined(_WIN32) && !defined(__CYGWIN__)
 		/*
 		 * archive_mstring on Windows directly convert multi-bytes
 		 * into archive_wstring in order not to depend on locale