Fix SNMP scanners on OS X/FreeBSD

Fixes the following error when running metasploit on OS X/FreeBSD:

![image](https://cloud.githubusercontent.com/assets/1184013/4492132/be1fc78c-4a3e-11e4-8f0b-dc7f7e030953.png)

Since the socket is already connected, we can just `write()` our data.

##### Verification steps

On an OS X or FreeBSD box:

    msf> use auxiliary/scanner/snmp/snmp_enum
    msf> set RHOSTS 192.168.1.1
    msf> run

- [ ] You should not see a `EISCONN` error