Allow hyphens and spaces in usernames and put the regex used to validate them in a config option.

• config_defaults_inc.php
  (user_login_valid_regex): new option giving a regex to use for validating
  new login names

• core/user_api.php
  (user_is_name_valid): use new config option (user_login_valid_regex) for
  regular expression

• lang/strings_english.txt
  (ERROR_USER_NAME_INVALID): add a sentence describing what the user name
  limitations are

git-svn-id: http://mantisbt.svn.sourceforge.net/svnroot/mantisbt/trunk@2160 <a class="text" href="/?p=mantisbt.git;a=object;h=f5dc347c">f5dc347c</a>-c33d-0410-90a0-b07cc1902cb9