Escaped percent char is mishandled in patsubst arguments

According to https://www.gnu.org/software/make/manual/make.html#Text-Functions,
"‘%’ characters in patsubst function invocations can be quoted with preceding backslashes (‘\’). "
Kati does not do that:
```
$ cat /tmp/a.mk
$(error $(patsubst %,\%/%,a b c))
$ make -f /tmp/a.mk
/tmp/a.mk:1: *** %/a %/b %/c. Stop.
$ ckati -f /tmp/a.mk
/tmp/a.mk:1: *** \a/% \b/% \c/%.
```