logger.h

Introduce a warning callback to the authz file parser API.

We need this to warn about the use of empty groups in authz files;
this is not an error and doesn't affect the authz file semantics,
but it's nice to be able to tell the user about it.

See issues #4794, #4802 and #4803.

* subversion/include/svn_repos.h
(svn_repos_authz_warning_func_t): New callback function type.
(svn_repos_authz_read4): New; API revision.
(svn_repos_authz_read3): Deprecated.
(svn_repos_authz_parse2): New; API revision.
(svn_repos_authz_parse): Deprecated.

* subversion/libsvn_repos/authz.h
(svn_authz__parse): Add warning function and baton parameters.
* subversion/libsvn_repos/authz.c
(authz_read): Add warning function and baton parameters.
Update calls to svn_authz__parse.
(svn_repos_authz_read4): Revised from svn_repos_authz_read3.
(svn_repos_authz_parse2): Revised from svn_repos_authz_parse.
* subversion/libsvn_repos/authz_parse.c
(struct ctor_baton_t): Add members warning_func and warning_baton.
(create_ctor_baton): Initialise these new members of the constructor baton.
(emit_parser_warning): New.
(SVN_AUTHZ_PARSE_WARN): New; wrapper macro for the above.
(array_insert_ace): Ignore and warn about the use of empty groups.
(svn_authz__parse): Update implementation to match prototype.
* subversion/libsvn_repos/deprecated.c
(svn_repos_authz_read3, svn_repos_authz_parse): Implement deprecated functions.

* subversion/mod_authz_svn/mod_authz_svn.c
(log_svn_message): New; replaces log_svn_error so that it's useful for
logging warnings as well.
(log_svn_error): Reimplement, calling log_svn_message.
(struct authz_warning_baton_t): New.
(log_authz_warning): New.
(get_access_conf): Set up an authz warning handler and baton, and call
svn_repos_authz_read4 instead of svn_repos_authz_read3.

* subversion/svnserve/logger.h
(logger__log_error): Make the 'err' parameter a pointer-to-const.
Update the docstring to say that the error is not cleared.
(logger__log_warning): New.
* subversion/svnserve/logger.c
(log_message): New; common base for logger__log_error and logger__log_message.
Also *do not* allocate 8k on the stack, use the logger pool, which gets
cleared at the end of the function.
(logger__log_error): Reimplement.
(logger__log_warning): Implement.
* subversion/svnserve/serve.c
(log_error): Make the error parameter const. Fix the docstring.
(log_warning): New.
(load_authz_config): Add warning function and baton parameters and fix pool
handling. Now calls svn_repos_authz_read4 instead of svn_repos_authz_read3.
(find_repos): Add warning function and baton parameters for load_authz_config.
(handle_authz_warning): New.
(construct_server_baton): Pass an authz warning handler and baton to find_repos.

* subversion/tests/cmdline/authz_tests.py
(group_member_empty_string): Fix docstring.
(empty_group): New test case.
(test_list): Run it.
* subversion/tests/cmdline/svnauthz_tests.py
(svnauthz_empty_group_test): Extend the @Issues decorator.
Add a check for the expected warning on stderr.