Add shellcode for RC4 decoding

Provided as a block to be included into stagers and/or decoder stubs.
Also included is a test shellcode that can be used for verifying that the
algorithm is compatible to Ruby's OpenSSL RC4 algorithm.
Add shellcode for RC4 bind and reverse stagers

Those stagers will encrypt the initial stage with a 128-bit RC4 key and
the stage length with a XOR key. Both keys are embedded in the stager.

This should provide good evasion capabilities in addition to some
protection against MITM reversing (if the stager is sent a different
route, like in an executable on an USB key).

Note that, from a cryptanalyst's standpoint, it is a bad idea to reuse the
same stager (or stagers with the same RC4 and XOR keys) more than once
since an identical key will result in an identical keystream and make
correlation attacks easy. But I doubt that matters in practice.

Also note that since communication after the initial statging is not
encrypted, these stagers should be used in combination with additional
encryption support in the payloads (like Meterpreter).
Add stager modules for RC4 bind and reverse stagers

See the commit message of my last commit for caveats.
Improve RC4 shellcode

ESI is not clobbered; no need to clear EDX as only DL is filled before and
it is overwritten before use.

Shellcodes in ruby modules not regenerated, but I guess you want to
regenerate them again anyway :-)
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+.
shameless plug for posterity in stager asm
Regenerate ruby modules

For shellcode changes (removed unneeded instruction) committed in
46a5c4f4bf56daaae5eae73d2f30f4acad8c4f23. Saves 2 bytes per shellcode.
Whitespace
Break up long lines and add magic encoding comment
Land #1223

This adds rc4-encrypting stagers for Windows.

[Closes #1223]