lib: allow exporting all logging functions to Lua

lib: allow exporting all logging functions to Lua

Add a function that will export FRR's logging functions into a Lua
table, and add that table to the table of your choice (usually _ENV).

For instance, to add logging to the global environment:

  lua_gettable(L, LUA_REGISTRYINDEX);
  lua_gettable(L, LUA_RIDX_GLOBALS);
  frrlua_export_logging(L);

Then the following functions are globally accessible to any Lua scripts
running with state L:

  - log.debug()
  - log.info()
  - log.notice()
  - log.warn()
  - log.error()

These are bound to zlog_debug, zlog_info, etc. They only take one string
argument for now but this shouldn't be an issue given Lua's builtin
facilities for formatting strings.

Signed-off-by: Quentin Young <qlyoung@cumulusnetworks.com>