* encoding.c (rb_enc_set_base): Add for setting base encoding
  with their names. this is internal function.

* template/encdb.h.tmpl: specify ENC_SET_BASE for second
  encodings in each encoding files.

* enc/encdb.c (rb_enc_set_base): add a declaration.
  (ENC_SET_BASE): ditto.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@24383 b2dd03c8-39d4-4d8f-98ff-823fe69b080e