Update Subversion and dependencies to 1.14.0 LTS.

MFC r344438 (by emaste):

svn: support building with WITH_PIE

Subversion builds and links against its own .a archives using local
rules, so did not benefit from with the WITH_PIE library support added
in r344179. Apply the same _pie suffix locally.

Reviewed by: cem
Differential Revision: https://reviews.freebsd.org/D19246

MFC r352156 (by imp):

Remove directory empty after r266735

MFC r357299 (by cem):

contrib/apr: Remove scope leak UB

In apr_vformatter, the variable buf was declared inside a limited scope
region, but a pointer to it is leaked outside of that region and used
later. This is undefined behavior. Fix by moving the buf variable to
function scope.

Reported by: Coverity
CID: 1192541

MFC r357301 (by cem):

contrib/apr: Rip out bogus [CS]PRNG implementation

This construction used some relatively slow design involving SHA2; even if
it were fed real entropy (unclear; external to the design), it did not
handle fork in a safe way, and it was difficult to audit for correctness.
So just rip it out and use the very simple and known-correct arc4random(3)
interface in its place.

MFC r361677:

Change Makefiles under usr.bin/svn to make them easier to incrementally
update. No functional change intended.

MFC r361678:

Update apr to 1.7.0. See contrib/apr/CHANGES for a summary of changes.

MFC r361681:

Change more Makefiles under usr.bin/svn to make them easier to
incrementally update. No functional change intended.

MFC r361684 (by kevans):

apr: build some needed emulated 64-bit atomic bits after r361678

This should fix the build on armv{6,7}, mips, and mips64, which all need
emulated 64-bit atomics for apr.

MFC r361691:

Follow-up r361678 (update apr to 1.7.0) by also regenerating the apr
internal headers through the upstream configure script, with some minor
custom tweaks.

MFC r361692:

Update apr-util to 1.6.1. See contrib/apr-util/CHANGES for a summary of
changes.

MFC r361693:

Update Subversion to 1.14.0 LTS. See contrib/subversion/CHANGES for a
summary of changes, or for a more thorough overview:

https://subversion.apache.org/docs/release-notes/1.14

Relnotes: yes