Fix bug in --echo for IPv6 ranges & add --echo-cidr option

[Frky]

This PR does 2 things:

• it aims to addresses Issue #677 -- see details below
• it adds a new CLI option --echo-cidr to output IP addresses at CIDR format (X.Y.Z.T/M) instead of range format (X1.Y1.Z1.T1-X2.Y2.Z2.T2) -- see details below

These two things are in one PR because they modify the same parts of the code - two different PR would lead to merge conflicts.

---

Address Issue #677

• fix bug in "small" CIDR ranges (/65../128)
• handle "large" CIDR ranges (/0../63)

$ bin/masscan --echo :: ::1
seed = 6069904213427194911
rate = 100       
shard = 1/1
nocapture = servername
nocapture = servername

# TARGET SELECTION (IP, PORTS, EXCLUDES)
ports = 
range = ::/127

$ bin/masscan --echo 2000:: 2000::1
seed = 2054565738792150746
rate = 100       
shard = 1/1
nocapture = servername
nocapture = servername

# TARGET SELECTION (IP, PORTS, EXCLUDES)
ports = 
range = 2000::/127

$ bin/masscan --echo 2000::/63
seed = 14647153754214998088
rate = 100       
shard = 1/1
nocapture = servername
nocapture = servername

# TARGET SELECTION (IP, PORTS, EXCLUDES)
ports = 
range = 2000::/63

$ bin/masscan --echo 2000::-2000:ffff:ffff:ffff:ffff:ffff:ffff:ffff
seed = 3141632608958804321
rate = 100       
shard = 1/1
nocapture = servername
nocapture = servername

# TARGET SELECTION (IP, PORTS, EXCLUDES)
ports = 
range = 2000::/16

---

Add --echo-cidr CLI option

If I am not mistaken, there is currently no option in masscan to output the list of IP ranges to be scanned:

• ordered by numerical order of the first IP of each range
• only using CIDR notations (<first ip>/<mask>).

Indeed, the option -sL outputs a list of IP addresses (not ranges) in a random order, and --echo outputs masscan configuration, including ranges but also other options, and in a non-consistent format.

This PR adds the option --echo-cidr that outputs the the ordered list of CIDR ranges. For instance, with the following list of IP addresses to scan:

10.0.0.0-10.0.1.15
192.168.0.0-192.168.0.3
192.168.1.2-192.168.1.3

• masscan -iL ips.txt --echo-cidr

  10.0.0.0/24
  10.0.1.0/28
  192.168.0.0/30
  192.168.1.2/31

• masscan -iL ips.txt -sL | head -n 10

  10.0.0.111
  10.0.0.61
  10.0.0.82
  192.168.0.3
  10.0.0.190
  10.0.0.214
  10.0.0.121
  10.0.0.42
  10.0.0.193
  10.0.1.4

• masscan -iL ips.txt --echo

  
  seed = 7008350106479622823
  rate = 100       
  shard = 1/1
  nocapture = servername
  nocapture = servername

TARGET SELECTION (IP, PORTS, EXCLUDES)

ports = range = 10.0.0.0-10.0.1.15 range = 192.168.0.0/30 range = 192.168.1.2/31