codecs: delete some unused codes

Delete GIF/JPEG/MNG support.
codecs: make PNG support mandatory

Make PNG support mandatory.
cleanup: assume ncurses, iconv & langinfo

Assume ncurses, iconv & langinfo are always there.
configure: clean up fbdev option's remains

Clean up fbdev option's remains.
README: update needed libraries

Let's try to list all the needed libraries in the README file.