pam-moduler: Add first implementation of a Go PAM Module generator

A PAM module can be generated using pam-moduler and implemented fully in
go without having to manually deal with the C setup.

Module can be compiled using go generate, so go:generate directives can be
used to make this process automatic, with a single go generate call as shown
in the example.

.codecov.yml

@@ -0,0 +1,3 @@
+ignore:
+  # Ignore pam-moduler generated files
+  - "**/pam_module.go"