Clarify start/end of AutoGen tracing

It is sometimes hard to match them up during a nested run

* agen5/ag-text.def: mark the start and end of tracing more clearly
* agen5/agUtils.c (open_trace_file): local function
  (check_make_dep_env): likewise
  (count_nl): move to defLex.c -- the only place used
  (skip_expr) DeCamelization
* agen5/defLex.c (count_nl): new
* agen5/test/debug.test: adjust for new start/end markers
* doc/Makefile.am: run_mktexi needs to specify the source directory