Bug 381: Make codepage data const.

Bug 381: Make codepage data const.

Before:

   text	   data	    bss	    dec	    hex	filename
  25726	  62992	   3343	  92061	  1679d	src/intl/charsets.o
 653856	 120020	  82144	 856020	  d0fd4	src/elinks

After:

   text	   data	    bss	    dec	    hex	filename
  60190	  28528	   3311	  92029	  1677d	src/intl/charsets.o
 688320	  85556	  82112	 855988	  d0fb4	src/elinks

So 34464 bytes were moved from the data section to the text section
and should be more likely to get shared between ELinks processes.

Measured on i686-pc-linux-gnu with: --disable-xbel --disable-nls
--disable-cookies --disable-formhist --disable-globhist
--disable-mailcap --disable-mimetypes --disable-smb --disable-mouse
--disable-sysmouse --disable-leds --disable-marks --disable-css
--enable-small --enable-utf-8 --without-gpm --without-bzlib
--without-idn --without-spidermonkey --without-lua --without-gnutls
--without-openssl CFLAGS="-O2 -ggdb -Wall"