Support for attributes specified multiple times

[jamesharr]

Certain attributes are specified multiple times in a RADIUS packet. A common example is Cisco-AVPair, but many network vendors do similar things.

Example data from pyrad:

Attributes returned form server (using `pkt.keys()` and `pkt[k]`):
 Service-Type: ['NAS-Prompt-User']
 Cisco-AVPair: ['shell:priv-lvl=15', 'shell:tasks=#root-system,#cisco-support']

Attributes returned form server (using `pkt.items()`):
 6: [b'\x00\x00\x00\x07']
 (9, 1): [b'shell:priv-lvl=15', b'shell:tasks=#root-system,#cisco-support']

Example data from radclient to the same radius server:

Sent Access-Request Id 114 from 0.0.0.0:52476 to REDACTED:1812 length 47
        User-Name = "REDACTED"
        User-Password = "REDACTED"
        Cleartext-Password = "REDACTED"
Received Access-Accept Id 114 from REDACTED:1812 to REDACTED:52476 length 98
        Service-Type = NAS-Prompt-User
        Cisco-AVPair = "shell:priv-lvl=15"
        Cisco-AVPair = "shell:tasks=#root-system,#cisco-support"