Shellcode syscall templates

[zachriggle]

Add syscall stub auto-generator, and auto-generated syscall stubs

Auto-generated syscall stubs are stored in pwnlib/shellcraft/common/linux/syscalls.

Each other architecture has a symlink to that directory at e.g. pwnlib/shellcraft/i386/linux.

This lets us be very explicit with shellcraft.i386.linux.syscalls.connect versus the more convenient shellcraft.connect (which would expand to e.g. shellcraft.i386.linux.connect).

Fixes #685

[TethysSvensson]

I don't have any opinion on the actual code (I have not looked at it), but could you make sure that this is either superior to #712 or close #712 after you are done?

Pinging @kristoff3r for an opinion on this.

[zachriggle]

@kristoff3r Are there any additional points that I need to pull in from your pull request?

[zachriggle]

CI is passing, but there are some remaining issues (also on the other PR)

• Need to fix Codacy issues
• Missing templates for socketcall
  • This is because none of the C headers have the function prototype. I have some local code that scrapes manpages for missing syscall, but need to upload it.
• We should really add new tests for all of the socket shellcode
• We should be able to enable the ELF stagers, but they are broken
  • We can greatly simplify the logic with memfd_create and execveat and atoi

[zachriggle]

I think that this is done now, @kristoff3r and @idolf.

Basically all of the logic is here:
https://github.com/zachriggle/pwntools/blob/shellcode-syscall-templates/pwnlib/data/syscalls/generate.py

The list of functions comes from here:
https://github.com/zachriggle/functions/blob/master/functions.py

Notable differences from #880 are being regenerate-able, supporting complex types, and having ALL THE SYSCALLS. I think that #880 is still missing some socket-related syscall stubs (e.g. no /syscalls/connect.asm for most architectures), and only has the "convenience" stubs (e.g. i386/linux/connect.asm).

Also, instead of a symlink-per-syscall, this one just symlinks the e.g. i386/linux/syscalls directory to common/linux/syscalls. Everything "magically" works out correctly because of context scoping.