Secure_log: add 'send_eof' parameter

Rsyslog reads the files and hangs up until it reads a predetermined
number of lines. This option makes secure_log send a EOF when the
buffer is empty, allowing rsyslog to flush its cache.
This option have security consequences: if someone else is reading
the socket at the same time, some data may not be read by rsyslog
Add a missing '__must_hold(log_lock)'
Security_log: security fix, potential buffer overflow at end of buffer

The reader might be positioned at the end of the buffer before
this recored is 'cleaned out' by a new record (if there is not enough
room remaining). However, when reading, the reader does not check if
the record was 'cleaned out' ("len" set to 0). This commit add such a check:
If the len of the current record is 0, go back to the beginning
of the buffer.
Keep a full sec_log instead of a single size_t at the end of the buffer
Use WARN_ON to detect malformed records and abort
Update version of secure_log module