rtnl: improve event reception in order to avoid ENOBUFS

[jow-]

• rtnl: increase event socket rx buffer size limit to 1 MiB
• rtnl: optimize reception of rtnl events
• rtnl: store callback in listener registry only on success

Fixes: #184

[f00b4r0]

AFAICT it does fix the breakage seen in #184 Thanks!