Fix recvuntil, and make it actually behave as the docs say

Currently it's *not* just a wrapper around `recvpred`.  With this patch, it is.

```py
from pwn import *

with open('example.py','w+') as e:
    e.write('print "HELLO"\n')
    e.write('print "WORLD"\n')
    e.write('print "DONTWANT"\n')

p = process(['python','example.py'])
data = p.recvuntil("WORLD")

assert data == "HELLO\nWORLD"
```