max_files_per_process ignored on Windows

At postmaster startup, we determine the maximum number of open files we 
can handle by trying to open a lot of file descriptors, up to 
max_files_per_process. This is done in set_max_safe_fds(), and the 
determined max_safe_fds value is inherited by child processes at fork(). 
However, with EXEC_BACKEND, ie. Windows, it's not inherited, so we 
always run with the initial conservative default of 32.

An obvious fix would be to call set_max_safe_fds() in the child 
processes, although I wonder if that's too expensive. Another option is 
to pass down the value with save_restore_backend_variables().

Thoughts? Although this has apparently always been like this, no-one has 
complained, so I'm thinking that we shouldn't backport this.

--   Heikki Linnakangas  EnterpriseDB   http://www.enterprisedb.com