chore: modularize PAM

also helps with sanitizing the code flow when spawning the child, more
code though

include/pam.h

@@ -0,0 +1,34 @@
+#ifndef PAM_H
+#define PAM_H
+
+#include <pwd.h>
+#include <security/_pam_types.h>
+#include <security/pam_appl.h>
+#include <stdbool.h>
+
+#include "macros.h"
+#include "sessions.h"
+
+#define PAMH_ERR_NOERR 0
+#define PAMH_ERR_ALLOC 1
+#define PAMH_ERR_ERRNO 2
+#define PAMH_ERR_NOERRNO 3
+
+struct pamh_getenv_status {
+  char error_flag;
+  union {
+    char** envlist;
+    const char* errfn;
+  };
+};
+
+// Doesn't include `source`s
+struct pamh_getenv_status pamh_get_complete_env(pam_handle_t* handle,
+                                                char* NNULLABLE user,
+                                                struct passwd* NNULLABLE pw,
+                                                enum session_type session_typ);
+
+void free_envlist(char** NNULLABLE envlist);
+pam_handle_t* NULLABLE get_pamh(char* NNULLABLE user, char* NNULLABLE passwd);
+
+#endif /* PAM_H */