Add the shelving v2 implementation from Subversion 1.11, as an alternative to the shelving v3 implementation from Subversion 1.12.
They have substantially different pros and cons, so it is beneficial for the user to be able to choose.
Make the shelving CLI version selectable by an environment variable: env. var. not set => shelving v3 enabled SVN_EXPERIMENTAL_COMMANDS=shelf3 => shelving v3 enabled SVN_EXPERIMENTAL_COMMANDS=shelf2 => shelving v2 enabled SVN_EXPERIMENTAL_COMMANDS= => no shelving CLI
* subversion/svn/svn.c Enable shelving v3 or v2 or neither, depending on the environment variable SVN_EXPERIMENTAL_COMMANDS.
* subversion/include/private/svn_client_shelf2.h * subversion/libsvn_client/shelf2.c * subversion/svn/shelf2-cmd.c * subversion/svn/shelf2-cmd.h * subversion/tests/cmdline/shelf2_tests.py New files, with contents copied from svn 1.11, with minor adjustments.