LN environment variables and resources

LN uses environment variables and resource variables.

Environment variables: These are operating system variables that have a meaning for LN. For example, BSE, BSE_TMP, and PATH.
Resource variables: These variables are not available in the operating system, but are only recognized within the context of LN. For example, high_ascii_tolerance, bshell_max_strbuf_size, and use_shm_info.

Various LN variables have a corresponding resource.