Raise default max-specpdl-size value

Raise default max-specpdl-size value

Occasionally, loading cl-generic.el from source requires
max_specpdl_size > 1600 when bootstrapping, and thus fails.
In any case we are very close to the limit.

* src/eval.c (init_eval_once): Raise max_specpdl_size to 1800.
* doc/lispref/variables.texi (Local Variables): Update docs.