output: cache the mapped file names

Don't repeatedly call malloc/free for each call to map_file_name.

* bootstrap.conf: We need hash-map.
* src/files.h, src/files.c (map_file_name): The caller must not free
the result.
Adjust callers.
(mapped_dir_prefix, spec_mapped_header_file): Remove.
* src/files.c
(map_file_name): Rename as...
(map_file_name_alloc): this.
(mapped_files, map_file_name, string_equals, string_hash, string_free):
New.

bootstrap.conf

@@ -27,6 +27,7 @@ gnulib_modules='
   getopt-gnu
   gettext-h git-version-gen gitlog-to-changelog
   gpl-3.0 intprops inttypes isnan javacomp-script
+  hash-map
   javaexec-script
   ldexpl
   libtextstyle-optional
@@ -58,7 +59,7 @@ gnulib_modules='
   xconcat-filename
   xhash
   xlist
-  xmemdup0
+  xmap xmemdup0
   xstrndup
 
   fprintf-posix printf-posix snprintf-posix sprintf-posix