BUG#58246: INSTALL PLUGIN not secure & crashable

When installing plugins, there is a missing check
for slash (/) in the path on Windows. Note that on
Windows, both / and \ can be used to separate
directories.

This patch fixes the issue by:
- Adding a FN_DIRSEP symbol for all platforms
  consisting of a string of legal directory
  separators.
- Adding a charset-aware version of strcspn().
- Adding a check_valid_path() function that uses
  my_strcspn() to check if any FN_DIRSEP character
  is in the supplied string.
- Using the check_valid_path() function in
  sql_plugin.cc and sql_udf.cc (which means
  replacing the existing test there).

sql/sql_plugin.h

@@ -131,6 +131,7 @@ extern bool mysql_uninstall_plugin(THD *thd, const LEX_STRING *name);
 extern bool plugin_register_builtin(struct st_mysql_plugin *plugin);
 extern void plugin_thdvar_init(THD *thd);
 extern void plugin_thdvar_cleanup(THD *thd);
+extern bool check_valid_path(const char *path, size_t length);
 
 typedef my_bool (plugin_foreach_func)(THD *thd,
                                       plugin_ref plugin,