xbps-install(1): added -I, --ignore-file-conflicts.

If set it will continue with the transaction and will
just print what are the conflicting files without returning
EEXIST.

This is a temporary solution for void where there are still
some packages with conflicting files (qt5-host-tools vs qt5-tools-devel).

With input by @duncaen

lib/transaction_files.c

@@ -460,6 +460,9 @@ collect_file(struct xbps_handle *xhp, const char *file, size_t size,
 			    EEXIST, pkgver,
 			    "%s: file `%s' already installed by package %s.",
 			    pkgver, file, item->new.pkgver);
+			if (xhp->flags & XBPS_FLAG_IGNORE_FILE_CONFLICTS)
+				return 0;
+
 			return EEXIST;
 		}
 		goto add;