Allow languages/strings.db to override builtin strings.

Allow languages/strings.db to override builtin strings.

src/helpfile.c (language_read): Add space before "parent".
  (language_read_list): Explicitly skip the file named "strings.db".
  (helpfile_read_languages): New function.
  (helpfile_finalize): Register it as a conf-reload function.
git-archimport-id: srvx@srvx.net--2006/srvx--devo--1.3--patch-67