Local debug runtimes; compile-time vs. runtime log levels check

A new family of entry points _l_ resp. _lb_ that retrieve debug runtimes via a call _get_local_debug_runtime () resp. _get_local_printbox_debug_runtime (): these functions are correspondingly expected in the scope. The entry points facilitate using thread-local (and domain-local) debug runtimes.

Compile-time vs. runtime consistency check for %%global_debug_log_level_from_env_var, and %%global_debug_log_level_from_env_var_unsafe that bypasses the check.