Make configure argument meaning more consistent and obvious.

Most other software uses --disable/--without for features it does build
or at least check by default.

Change aria2 configure options so that:
 * --enable-*: do not build by default, unless --enable specified
 * --disable-*: check and build by default, unless --disable specified
 * --with-*: do not use by default, unless --with specified
 * --without-*: check and use by default, unless without specified
Merge branch 'clear-configure' of https://github.com/nmaier/aria2 into nmaier-clear-configure
aria2_arg.m4: Doc fix and use default=yes instead of check