lib/util_hash.c: add xbps_file_hash_raw method

this function does not mmap the target file and therefore
avoids out of memory exceptions on 32bit systems.

include/xbps.h.in

@@ -1795,6 +1795,17 @@ bool xbps_mmap_file(const char *file, void **mmf, size_t *mmflen, size_t *filele
  */
 char *xbps_file_hash(const char *file);
 
+/**
+ * Returns a raw byte buffer with the sha256 hash for the file specified
+ * by \a file.
+ *
+ * @param[in] file Path to a file.
+ * @return A pointer to a malloc(3)ed buffer, NULL otherwise and errno
+ * is set appropiately. The pointer should be free(3)d when it's no
+ * longer needed.
+ */
+unsigned char *xbps_file_hash_raw(const char *file);
+
 /**
  * Compares the sha256 hash of the file \a file with the sha256
  * string specified by \a sha256.