replace charset.def with generated yaws_charset.hrl

Richard Carlsson pointed out in a private email that having configure
generate the priv/charset.def file to be read and interpreted by
src/mime_type_c.erl was overly complicated.

Modify configure script and rebar.config.script to instead generate
src/yaws_charset.hrl, and include that into src/mime_type_c.erl. Thanks to
Richard for suggesting these simplifications.

Also, enhance rebar.config.script to be able to extract the desired charset
from the YAWS_CHARSET OS environment variable if set, thus providing rebar
users a way to set the charset, which they couldn't do before.

Fix test/t2/app_test.erl to not fail on Content-Type header tests if the
header value contains a charset specification.