This commit adds DNS resolution to rev_tcp_rc4

Due to the modular structure of payload stages its pretty trivial
to add DNS resolution instead of hard-coded IP address in stage0.

The only real complication here is that ReverseConnectRetries ends
up being one byte further down than in the original shellcode. It
appears that the original rev_tcp_dns payload suffers from the same
issue.

Hostname substitution is handled in the same method as the RC4 and
XOR keys, with an offset provided and replace_vars ignoring the
hostname.

Tested in x86 native and WOW64 on XP and 2k8r2 respectively.

This is a good option for those of us needing to leave persistent
binaries/payloads on hosts for long periods. Even if the hostname
resolves to a malicious party attempting to steal our hard earned
session, they'd be hard pressed to crypt the payload with the
appropriate RC4 pass. So long as we control the NS and records, the
hardenned shellcode should provide a better night's sleep if running
shells over the WAN. Changing the RC4 password string in the
shellcode and build.py should reduce the chances of recovery by RE.

Next step will likely be to start generating elipses for ECDH SSL
in meterpreter sessions and passing them with stage2 through the
RC4 socket. If P is 768-1024 the process is relatively quick, but
we may want to precompute a few defaults as well to have 2048+.