transaction: Define C functions as unexported static inlines

This will make it easier to avoid exporting unexpected symbols to the
generated PAM libraries.

Also it makes less messy handling C code inside go files.

transaction.h

@@ -0,0 +1,69 @@
+#include <security/pam_appl.h>
+#include <stdint.h>
+#include <stdlib.h>
+#include <string.h>
+
+#ifdef PAM_BINARY_PROMPT
+#define BINARY_PROMPT_IS_SUPPORTED 1
+#else
+#include <limits.h>
+#define PAM_BINARY_PROMPT INT_MAX
+#define BINARY_PROMPT_IS_SUPPORTED 0
+#endif
+
+#ifdef __sun
+#define PAM_CONST
+#else
+#define PAM_CONST const
+#endif
+
+extern int _go_pam_conv_handler(struct pam_message *, uintptr_t, char **reply);
+
+static inline int cb_pam_conv(int num_msg, PAM_CONST struct pam_message **msg, struct pam_response **resp, void *appdata_ptr)
+{
+	if (num_msg <= 0 || num_msg > PAM_MAX_NUM_MSG)
+		return PAM_CONV_ERR;
+
+	*resp = calloc(num_msg, sizeof **resp);
+	if (!*resp)
+		return PAM_BUF_ERR;
+
+	for (size_t i = 0; i < num_msg; ++i) {
+		int result = _go_pam_conv_handler((struct pam_message *)msg[i], (uintptr_t)appdata_ptr, &(*resp)[i].resp);
+		if (result != PAM_SUCCESS)
+			goto error;
+	}
+
+	return PAM_SUCCESS;
+error:
+	for (size_t i = 0; i < num_msg; ++i) {
+		if ((*resp)[i].resp) {
+			memset((*resp)[i].resp, 0, strlen((*resp)[i].resp));
+			free((*resp)[i].resp);
+		}
+	}
+
+	memset(*resp, 0, num_msg * sizeof *resp);
+	free(*resp);
+	*resp = NULL;
+	return PAM_CONV_ERR;
+}
+
+static inline void init_pam_conv(struct pam_conv *conv, uintptr_t appdata)
+{
+	conv->conv = cb_pam_conv;
+	conv->appdata_ptr = (void *)appdata;
+}
+
+// pam_start_confdir is a recent PAM api to declare a confdir (mostly for
+// tests) weaken the linking dependency to detect if it’s present.
+int pam_start_confdir(const char *service_name, const char *user, const struct pam_conv *pam_conversation,
+		      const char *confdir, pam_handle_t **pamh) __attribute__((weak));
+
+static inline int check_pam_start_confdir(void)
+{
+	if (pam_start_confdir == NULL)
+		return 1;
+
+	return 0;
+}