New script sanitize-schem to remove subsheets for which no file can be found.

- schhist/sanitize-schem: remove inaccssible schematics sub-sheets
- schhist/gitsch2ps (usage, "main"): -S runs sanitize-schem in addition to
  sanitize-profile
- schhist/schhist2web (usage): -S now also sanitizes the top-level sheet

schhist/schhist2web

@@ -118,7 +118,7 @@ usage: $0 [-c cache-dir] [-f] [-n] [-S] [top-dir] [top-schem] [out-dir]
   -c cache-dir  cache directory (default: same as out-dir)
   -f            identify sheets by their file name, not the sheet name
   -n            don't use previous cache content (rebuild the cache)
-  -S            sanitize KiCad profile
+  -S            sanitize KiCad profile and top-level sheet
 EOF
     exit 1
 }