BUG/MINOR: Fix several leaks of 'log_tag' in init().

BUG/MINOR: Fix several leaks of 'log_tag' in init().

We use chunk_initstr() to store the program name as the default log-tag.

If we use the log-tag directive in the config file, this chunk will be
destroyed and replaced. chunk_initstr() sets the chunk size to 0 so we
will free the chunk itself, but not its content.

This happens for a global section and also for a proxy.

We fix this by using chunk_initlen() instead of chunk_initstr().
We also check that the memory allocation was successfull, otherwise we quit.

This fixes github issue #850.
It can be backported as far as 1.9, with minor adjustments to includes.