Following up on a suggestion by Ivan on @dev about a month ago.

This is an initial draft plus completely untested implementation
for an SVN file API.  Its implementation works around a few
inefficiencies and scalability issues with apr_file_t.  See the
header comments for details.

Tests will be added eventually and the code stabilized based on
the test results.  To prevent accidental use in current production
code, the new API is only available if you define #ENABLE_SVN_FILE
in your code.

* subversion/include/private/svn_file.h
  (): new file, declaring the svn_file_t API.

* subversion/libsvn_subr/file.c
  (): implement new API; missing documentation

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1517733 13f79535-47bb-0310-9956-ffa450edef68