Racy einfo allocation and emlog_info_list manipulation

[ahippo]

As mentioned in a comment for #7, there are a few other race conditions:

• with emlog_info_list manipulation (create_einfo() vs free_einfo())
• with new einfo allocation (get_einfo() vs create_einfo())

This may cause memory leaks or crashes during concurrent opening of new emlog buffers,
or concurrent closing/opening of an emlog device (when loaded with emlog_autofree=1 (defaults to off)).