Added support for macOS, additional IP formats, and to drop privileges

[acjordan2]

Changes Submitted:

• [Security] Application will drop privileges by default if running as the root account.

  • By default the user is nobody but this can be configured via the -dropToUsername <username> argument or disabled via the -dontDropPrivileges.

• [Bug Fix] Application now compiles and runs on macOS.

  • The TProxy syscall does not appear to have an equivalent on macOS. Moving this feature to its own platform specific file, singularity_linux.go, allows singularity to be compiled and run on macOS.

• [Feature] Added support for base 10 encoded IP addresses.

  • I ran into a case where I had an SSRF that was also vulnerable to DNS rebinding, however, exploitation limited the amount of subdomains I was able to use. I was able to get around this by patching singularity to also allow base 10 encoded IP address. The original functionality is unchanged. e.g: s-8.8.8.8-127.0.0.1-1-fs-e.dynamic.example.com becomes s-134744072-2130706433-1-fs-e.dynamic.example.com

[sanktjodel]

Thank you for your contribution. We will consider merging each feature individually in future improvements.