Fix issue #2324: Disable username assumption

Patch by: Jeff Weiss <svn@jeffweiss.org>
Suggested by: chrisjensen

* Added the --prompt-username option to the command line client which
  ignores the os, cached, and --username usernames and always prompts
  for the username.

* subversion/include/svn_cmdline.h
  (svn_cmdline_setup_auth_baton): added boolean parameter for overriding
  os, cached, and provided usernames.

* subversion/include/svn_auth.h: added property for prompting of usernames

* subversion/libsvn_subr/cmdline.c
  (svn_cmdline_setup_auth_baton): added boolean parameter for overriding
  os, cached, and provided usernames, sets the svn_auth property if parameter
  is true.

* subversion/libsvn_subr/username_providers.c
  (username_first_creds): added test for existance of username prompt property.
  added additional condition based on that property to blocks getting cached
  and os usernames.

* subversion/libsvn_subr/simple_providers.c
  (simple_first_creds_helper): added test for existance of username prompt
  property.  Added additional condition based on that property to blocks
  getting cached and os usernames.
  (simple_prompt_first_creds): added test for existance of username prompt
  property.  Changed parameter of call to prompt_for_simple_creds to utilize
  existing functionality of prompting for username on 2nd+ tries.

* subversion/clients/cmdline/cl.h
  (svn_cl__longopt_t): added the prompt-username option to the long options
  enumeration in the appropriate alphabetical position.
  (svn_cl__opt_state_t): added boolean member for holding whether or not the
  --prompt-username option was passed.

* subversion/clients/cmdline/main.c
  (svn_cl__options): added the command line option of --prompt-username to 
  the array of possible options.
  (SVN_CL__AUTH_OPTIONS): added the svn_cl__longopt_t version (see
    subversion/clients/cmdline/cl.h) of the prompt username option
    to the authentication options.
  (main): added case to set the prompt username field of opt_state if the
  prompt-username command line option passed.  Added the opt_state field to
  the call to svn_cmdline_setup_auth_baton.