ebuild-writing/functions: Don't allow network access

There have been a number of bugs filed to remove network access from
ebuild phases [1,2,...], but until now this policy was undocumented
here.  We'll have to relax this restriction if the src_fetch [3,4,5] or
pkg_fetch [6] proposals being discussed for EAPI-6 are accepted, but
neither are currently mentioned in either the PMS [7] or Portage [8].

Yesterday on #gentoo-dev-help:

  16:12 < wking> Can anyone point me to the policy docs for not
    allowing network access (outside of SRC_URI and similar, e.g. from
    src_compile) in ebuilds?  I understand the reason for this, but
    I'd like to link to the official policy from a commit message in
    my overlay.
  16:26 <@mrueg> wking: i think it is just missing here:
    https://devmanual.gentoo.org/ebuild-writing/functions/src_test/index.html
  16:28 < wking> mrueg: so it's not explicitly documented yet?
  16:29 <@mrueg> wking: at least i haven't found it 

And in #gentoo-python:

  16:40 < idella4> prometheanfire: where the **** does it say in the
    manual no net rule for beyond src_fetch?????
  16:40 < jbergstroem> idella4: do you mean as in no downloading
    stuff? That's been around forever. lemme see if i can find a doc
    reference about it
  16:40 < jbergstroem> idella4: there's been a lot of bugs filed
  16:42 < idella4> thanks jberg
  16:42 < idella4> yes that's the one
  16:43 < jbergstroem> nor sure why it isn't; but it should probably live here 
    https://devmanual.gentoo.org/ebuild-writing/functions/index.html
  16:44 < jbergstroem> and also refr to pkg_fetch
  …
  19:39 < jbergstroem> wking: the devmanual could still benefit from
    being clearer about how/when to fetch sources.  Especially since
    newer packages try to automate suff like this (invoking wget
    from make or whatever for their deps)

[1]: https://bugs.gentoo.org/show_bug.cgi?id=315403
     Summary: media-video/jubler fails to build without network
[2]: https://bugs.gentoo.org/show_bug.cgi?id=336771
     Summary: dev-libs/libevent tests require network access
[3]: https://bugs.gentoo.org/show_bug.cgi?id=182028
     Summary: [Future EAPI] About managing CVS/SUBVERSION version of
       software
[4]: http://thread.gmane.org/gmane.linux.gentoo.devel/47645/focus=52918
     From: Marijn Schouten <hkBst@gentoo.org>
     Subject: src_fetch (was Re: EAPI 1 (Was: Re: Monthly Gentoo
       Council Reminder for April))
     Date: 2007-11-09 17:41:38 GMT
[5]: http://thread.gmane.org/gmane.linux.gentoo.devel/87869
     From: Michał Górny <mgorny@gentoo.org>
     Subject: [EAPI 6] src_fetch() for fetching live sources
     Date: 2013-08-27 10:01:27 GMT
[6]: http://thread.gmane.org/gmane.linux.gentoo.devel/87869/focus=878773
     From: Ian Stakenvicius <axs@gentoo.org>
     Subject: Re: [EAPI 6] src_fetch() for fetching live sources
     Date: 2013-08-27 14:33:27 GMT
[7]: git://git.overlays.gentoo.org/proj/pms.git
[8]: git://git.overlays.gentoo.org/proj/portage.git