Use integer backend type when exec'ing a postmaster child

Use integer backend type when exec'ing a postmaster child

This way we don't have to walk the entire process type array and
strcmp() the string with the names therein.  The integer value can be
directly used as array index instead.

Author: Álvaro Herrera <alvherre@kurilemu.de>
Reviewed-by: Euler Taveira <euler@eulerto.com>
Discussion: https://postgr.es/m/202512090935.k3xrtr44hxkn@alvherre.pgsql