libremail: Add shared try_mkdir() function

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

libremail/common/file_mkdir.c

@@ -0,0 +1,24 @@
+/*
+ * Copyright (c) 2026, Chloe Moffett
+ * Provided under the BSD-3 clause
+ */
+
+#include <sys/stat.h>
+#include <unistd.h>
+#include <errno.h>
+#include "libremail/file.h"
+
+int
+try_mkdir(const char *path, mode_t mode)
+{
+    if (path == NULL) {
+        errno = EINVAL;
+        return -1;
+    }
+
+    if (access(path, F_OK) != 0) {
+        return mkdir(path, mode);
+    }
+
+    return 0;
+}