Rework .stignore functionality (fixes #561) (...)

- Only one .stignore is supported, at the repo root
 - Negative patterns (!) are supported
 - Ignore patterns affect sent and received indexes, not only scanning

scanner/testdata/excludes

@@ -1,4 +1,2 @@
 dir2/dfile
-#include excludes
 #include further-excludes
-#include loop-excludes