Introduce debug tags and debug breaks

The environment variable DEBUG_TAGS can be used to control what is
printed on stderr. Example:

    DEBUG_TAGS="+all,-Archiver.*" ./restic backup foo

The variable DEBUG_BREAK can be used to set a breakpoint (implemented
using SIGSTOP):

    DEBUG_BREAK=Archiver.Snapshot ./restic backup foo

debug_release.go

@@ -2,4 +2,6 @@
 
 package restic
 
-func debug(fmt string, args ...interface{}) {}
+func debug(tag string, fmt string, args ...interface{}) {}
+
+func debug_break(string) {}