[MINOR] Capture & display more data from health checks, v2

[MINOR] Capture & display more data from health checks, v2

Capture & display more data from health checks, like
strerror(errno) for L4 failed checks or a first line
from a response for L7 successes/failed checks.

Non ascii or control characters are masked with
chunk_htmlencode() (html stats) or chunk_asciiencode() (logs).