fakedns improvements to more properly support SRV record handling in BYPASS and FAKE modes

This addresses most of my feedback in rapid7/metasploit-framework#4067.  

## Validation
 - [ ] Validate that when ```BYPASS```, a mostly correct response is returned:
```
resource (/tmp/fakedns.rc)> set TARGETACTION BYPASS
TARGETACTION => BYPASS
resource (/tmp/fakedns.rc)> set TARGETDOMAIN _jabber._tcp.google.com
TARGETDOMAIN => _jabber._tcp.google.com
resource (/tmp/fakedns.rc)> run -j
[*] Auxiliary module running as background job
resource (/tmp/fakedns.rc)> dig srv _jabber._tcp.google.com @localhost +nostats +noquestion
[*] exec: dig srv _jabber._tcp.google.com @localhost +nostats +noquestion

[*] DNS server initializing
[*] DNS server started
[*] 127.0.0.1:41412 - DNS - XID 14812 (IN::SRV _jabber._tcp.google.com)

; <<>> DiG 9.8.1-P1 <<>> srv _jabber._tcp.google.com @localhost +nostats +noquestion
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 14812
;; flags: qr rd ra; QUERY: 1, ANSWER: 5, AUTHORITY: 0, ADDITIONAL: 5

;; ANSWER SECTION:
_jabber._tcp.google.com. 10	IN	SRV	5 0 5269 xmpp-server.l.google.com.
_jabber._tcp.google.com. 10	IN	SRV	20 0 5269 alt2.xmpp-server.l.google.com.
_jabber._tcp.google.com. 10	IN	SRV	20 0 5269 alt4.xmpp-server.l.google.com.
_jabber._tcp.google.com. 10	IN	SRV	20 0 5269 alt3.xmpp-server.l.google.com.
_jabber._tcp.google.com. 10	IN	SRV	20 0 5269 alt1.xmpp-server.l.google.com.

;; ADDITIONAL SECTION:
xmpp-server.l.google.com. 60	IN	A	74.125.22.125
alt2.xmpp-server.l.google.com. 60 IN	A	74.125.138.125
alt4.xmpp-server.l.google.com. 60 IN	A	173.194.65.125
alt3.xmpp-server.l.google.com. 60 IN	A	64.233.167.125
alt1.xmpp-server.l.google.com. 60 IN	A	64.233.186.125
```
 - [ ] Validate that when ```FAKE```, ```TARGETHOST``` is correctly used when not set:
```
resource (/tmp/fakedns.rc)> set TARGETACTION FAKE
TARGETACTION => FAKE
resource (/tmp/fakedns.rc)> set TARGETDOMAIN _jabber._tcp.google.com
TARGETDOMAIN => _jabber._tcp.google.com
resource (/tmp/fakedns.rc)> run -j
[*] Auxiliary module running as background job
resource (/tmp/fakedns.rc)> dig srv _jabber._tcp.google.com @localhost +nostats +noquestion
[*] exec: dig srv _jabber._tcp.google.com @localhost +nostats +noquestion

[*] DNS server initializing
[*] DNS server started
[*] 127.0.0.1:39642 - DNS - XID 9390 (IN::SRV _jabber._tcp.google.com)

; <<>> DiG 9.8.1-P1 <<>> srv _jabber._tcp.google.com @localhost +nostats +noquestion
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 9390
;; flags: qr rd ra; QUERY: 1, ANSWER: 5, AUTHORITY: 1, ADDITIONAL: 5

;; ANSWER SECTION:
_jabber._tcp.google.com. 10	IN	SRV	20 0 5269 alt1.xmpp-server.l.google.com.
_jabber._tcp.google.com. 10	IN	SRV	5 0 5269 xmpp-server.l.google.com.
_jabber._tcp.google.com. 10	IN	SRV	20 0 5269 alt2.xmpp-server.l.google.com.
_jabber._tcp.google.com. 10	IN	SRV	20 0 5269 alt4.xmpp-server.l.google.com.
_jabber._tcp.google.com. 10	IN	SRV	20 0 5269 alt3.xmpp-server.l.google.com.

;; AUTHORITY SECTION:
_jabber._tcp.google.com. 60	IN	NS	dns._jabber._tcp.google.com.

;; ADDITIONAL SECTION:
alt1.xmpp-server.l.google.com. 60 IN	A	127.0.0.1
xmpp-server.l.google.com. 60	IN	A	127.0.0.1
alt2.xmpp-server.l.google.com. 60 IN	A	127.0.0.1
alt4.xmpp-server.l.google.com. 60 IN	A	127.0.0.1
alt3.xmpp-server.l.google.com. 60 IN	A	127.0.0.1
```
 - [ ] Validate that when ```FAKE```, ```TARGETHOST``` is correctly used when set:
```
resource (/tmp/fakedns.rc)> set TARGETACTION FAKE
TARGETACTION => FAKE
resource (/tmp/fakedns.rc)> set TARGETDOMAIN _jabber._tcp.google.com
TARGETDOMAIN => _jabber._tcp.google.com
resource (/tmp/fakedns.rc)> set TARGETHOST 1.2.3.4
TARGETHOST => 1.2.3.4
resource (/tmp/fakedns.rc)> run -j
[*] Auxiliary module running as background job
resource (/tmp/fakedns.rc)> dig srv _jabber._tcp.google.com @localhost +nostats +noquestion
[*] exec: dig srv _jabber._tcp.google.com @localhost +nostats +noquestion

[*] DNS server initializing
[*] DNS server started
[*] 127.0.0.1:50653 - DNS - XID 34953 (IN::SRV _jabber._tcp.google.com)

; <<>> DiG 9.8.1-P1 <<>> srv _jabber._tcp.google.com @localhost +nostats +noquestion
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 34953
;; flags: qr rd ra; QUERY: 1, ANSWER: 5, AUTHORITY: 1, ADDITIONAL: 5

;; ANSWER SECTION:
_jabber._tcp.google.com. 10	IN	SRV	20 0 5269 alt3.xmpp-server.l.google.com.
_jabber._tcp.google.com. 10	IN	SRV	20 0 5269 alt1.xmpp-server.l.google.com.
_jabber._tcp.google.com. 10	IN	SRV	5 0 5269 xmpp-server.l.google.com.
_jabber._tcp.google.com. 10	IN	SRV	20 0 5269 alt2.xmpp-server.l.google.com.
_jabber._tcp.google.com. 10	IN	SRV	20 0 5269 alt4.xmpp-server.l.google.com.

;; AUTHORITY SECTION:
_jabber._tcp.google.com. 60	IN	NS	dns._jabber._tcp.google.com.

;; ADDITIONAL SECTION:
alt3.xmpp-server.l.google.com. 60 IN	A	1.2.3.4
alt1.xmpp-server.l.google.com. 60 IN	A	1.2.3.4
xmpp-server.l.google.com. 60	IN	A	1.2.3.4
alt2.xmpp-server.l.google.com. 60 IN	A	1.2.3.4
alt4.xmpp-server.l.google.com. 60 IN	A	1.2.3.4
```