MINOR: log/applet: add new function syslog_applet_append_event()

MINOR: log/applet: add new function syslog_applet_append_event()

This function takes a buffer on input, and offset and a length, and
consumes the block from that buffer to send it to the appctx's output
buffer. Contrary to its sibling applet_append_line(), instead of just
appending an LF at the end of the line, it prepends the message size
in decimal and a space before the message, as expected by syslog TCP
implementaions. This will be used to simplify the ring reader code.