Gentoo ebuild fixes

Several ebuild fixes:

- fixed portage warning `Repository 'dnscrypt-proxy' is missing masters attribute`
- updated libsodium ebuild to 0.4.5
- backported configure.ac patch for dnscrypt-proxy 1.3.3
- reworked ebuilds to use autotools instead of supplied ./configure

Since both projects use autotools I thought that it makes more sense to use the supplied autotools-utils eclass from Gentoo since it is a source-based distribution anyways and portage may be able to handle some edge cases this way. Also, the libsodium-ebuild is annoyingly simple now ;)

I noticed that dnscrypt-proxy does not support out of source builds because there are some files that are moved around with `cat`, I'll look into that later.

I still have to look into the rules and obligations, but I'd like to get these ebuilds in the official tree, or at least the sunrise overlay, in the near future. I guess that would mean that I'll be the "official" maintainer for them. Do you have any objections?