New Unix utility function to make a directory path.

Essentially 'mkdir -p' - we try to make each prefix of the pathname,
terminating on any error other than EEXIST. Semantics are similar to
make_dir_and_check_ours(): we return NULL on success or a dynamically
allocated error message string on failure.

unix/unix.h

@@ -11,6 +11,7 @@
 #include <dlfcn.h>		       /* Dynamic library loading */
 #endif /*  NO_LIBDL */
 #include "charset.h"
+#include <sys/types.h>         /* for mode_t */
 
 #ifdef OSX_GTK
 /*
@@ -197,6 +198,7 @@ void noncloexec(int);
 int nonblock(int);
 int no_nonblock(int);
 char *make_dir_and_check_ours(const char *dirname);
+char *make_dir_path(const char *path, mode_t mode);
 
 /*
  * Exports from unicode.c.

unix/uxmisc.c

@@ -322,3 +322,28 @@ char *make_dir_and_check_ours(const char *dirname)
 
     return NULL;
 }
+
+char *make_dir_path(const char *path, mode_t mode)
+{
+    int pos = 0;
+    char *prefix;
+
+    while (1) {
+        pos += strcspn(path + pos, "/");
+
+        if (pos > 0) {
+            prefix = dupprintf("%.*s", pos, path);
+
+            if (mkdir(prefix, mode) < 0 && errno != EEXIST) {
+                char *ret = dupprintf("%s: mkdir: %s",
+                                      prefix, strerror(errno));
+                sfree(prefix);
+                return ret;
+            }
+        }
+
+        if (!path[pos])
+            return NULL;
+        pos += strspn(path + pos, "/");
+    }
+}