libremail+tools: Add mailbox_create() function

It is best to prefer mailbox_create() rather than try_mkdir() directly
as we'll be able to encapslate mailbox creation logic within it.

Signed-off-by: Chloe M. <chloe@mirocom.org>

libremail/common/mailbox.c

@@ -0,0 +1,29 @@
+/*
+ * Copyright (c) 2026, Chloe Moffett
+ * Provided under the BSD-3 clause
+ */
+
+#include <stdint.h>
+#include <stdio.h>
+#include <stddef.h>
+#include <errno.h>
+#include "libremail/mailbox.h"
+
+int
+mailbox_create(const char *path, mode_t mode)
+{
+    int error;
+
+    if (path == NULL) {
+        errno = EINVAL;
+        return -1;
+    }
+
+    error = mkdir(path, mode);
+    if (error < 0) {
+        perror("mkdir");
+        return -1;
+    }
+
+    return 0;
+}