BUG/MINOR: admin: haproxy-reload use explicit socat address type

BUG/MINOR: admin: haproxy-reload use explicit socat address type

socat was used with the ${MASTER_SOCKET} variable directly, letting it
auto-detect the network protocol. However, when given a plain filename
that does not point to a UNIX socket, socat would create a file at that
path instead of reporting an error.

To fix this, the address type is now determined explicitly: if
MASTER_SOCKET points to an existing UNIX socket file (checked with -S),
UNIX-CONNECT: is used; if it matches a <host>:<port> pattern, TCP: is
used; otherwise an error is reported. The socat_addr variable is also
properly scoped as local to the reload() function.

Could be backported in 3.3.