build: Require Guile >= 2.0.13.

build: Require Guile >= 2.0.13.

* README, configure.ac, doc/guix.texi (Requirements): Increase minimum
Guile version from 2.0.9 to 2.0.13.
* config-daemon.ac: Remove use of 'GUIX_CHECK_UNBUFFERED_CBIP'.
* m4/guix.m4 (GUIX_CHECK_UNBUFFERED_CBIP): Remove.
* guix/build/download.scm (current-http-proxy): Remove.
* guix/build/syscalls.scm (%libc-errno-pointer, errno): Remove.
(syscall->procedure): Use #:return-errno unconditionally.
* guix/hash.scm (open-sha256-input-port)[unbuffered]: Remove outdated
comment.
* guix/http-client.scm (when-guile<=2.0.5-or-otherwise-broken): Remove.
<top level>: Remove 'when-guile<=2.0.5-or-otherwise-broken' block.
* guix/scripts/substitute.scm (fetch): Remove 'guile-version>?'
conditional.
* tests/hash.scm (supports-unbuffered-cbip?): Remove.
<top level>: Remove 'test-skip' call.