[MINOR] cfgparse: Check whether the path given for the stats socket actually fits...

[MINOR] cfgparse: Check whether the path given for the stats socket actually fits into the sockaddr_un structure to avoid truncation.

while working further on the changes to allow for dynamic
adding/removing of backend servers we noticed a potential problem: the
path given for the 'stats socket' global option may get truncated when
copying it into the sockaddr_un.sun_path field.

Attached patch checks the length, and reports an error if truncation
would happen.

This issue was noticed by Joerg Sonnenberger <joerg@NetBSD.org>.