Add nasl function sftp_enabled_check()

[jjnicola]

What:
Add nasl function sftp_enabled_check() to check if sftp subsystem is enabled in the target

Jira: SC-376

Why:
To check if sftp subsystem is enabled in the target

How:
Create the following nasl script sftp_test.nasl:

port = 22;
user = 'user';
pass = 'pass';

#Socket

soc1 = open_sock_tcp( port );
if( ! soc1 ) exit( 0 );
display ("socket opened:",soc1);

# New session
sess1 = ssh_connect( socket:soc1 );
display("session1: ", sess1);

# User auth.
ssh_userauth(sess1, login:user, password:pass);

#Check SFTP
ret = sftp_enabled_check (sess1);

display("SFTP: ", ret);

# Close session
a = ssh_disconnect(sess1);

Disable SFTP subsystem in the target

• Comment out sftp support in sshd_config and of course restart sshd:
  #Subsystem sftp /usr/lib/openssh/sftp-server

Run the script:
openvas-nasl -X -B -d -i /path/to/var/lib/openvas/plugins -t <TARGET> sftp_test.nasl

Re-enable SFTP and re run the test.

Checklist:

• Tests
• CHANGELOG Entry