fix build on openbsd

add remove_all_sinks method to dist_sink

This allows users to set exactly the sinks they want, even if other
unknown application code has added bespoke sinks in the meantime.

Closes #717

errno code is no longer OS specific. Moving to common.

This fix uses the fmt::safe_strerror instead of the underlying
strerror_r functionality. I think this is better - but I'm not
sure if there is a reason for avoiding the use of this function.

Failure to compile to systems that don't have a valid definition
of strerror_r.

Closes gabime/spdlog#74

Update step logger

Support for AIX. Tested on AIX 6.1 with GCC 6.3.

Fixed bugs with multithreading and added protection against data lost with insufficiently accurate setting of the maximum value of the file size

Fixed adding a file header and adding ignoring of empty files

Added file header support for step_logger