Deref watcher before trying to its max_size property

The netsnmp_watcher_info structure returned by netsnmp_create_watcher_info is
a pointer, so it must be de-referenced with ctypes' "contents" attribute
before any of its properties can be set.

Reported by Max "mk23" Kalika <max.kalika+projects@gmail.com>.
Fix comment added in 60d51c93
Use WATCHER_MAX_SIZE instead of WATCHER_SIZE_STRLEN for 5.4.x compatibility

We used to rely on net-snmp >= 5.5 behavior by setting a WATCHER_SIZE_STRLEN
flag that didn't exist in net-snmp 5.4.x yet. For backwards compatibility we
now use WATCHER_MAX_SIZE and update the used netsnmp_watcher_info structure's
"data_size" field ourselves.

Reported by Max "mk23" Kalika <max.kalika+projects@gmail.com>.
Update README with net-snmp compatibility remarks
Update README with new credits