Replace strdupa with more portable ast_strdupa

The strdupa function is a GNU extension, and not widely portable. We
have an ast_strdupa function used within Asterisk which is preferred.
I pulled the definition up from menuselect.c into the menuselect.h
header file so it can be shared across menuselect.

Change-Id: I9593c97f78386b47dc1e83201e80cb2f62b36c2e