[PATCH] Windows port: add support to setenv function

According to [1], windows does not support setenv.
With the possibility of setenv going further [2], I am submitting in this thread, the patch to add setenv support on the windows side,
It is based on pre-existing functions, and seeks to correctly emulate the functioning of the POSIX setenv, but has not yet been tested.regards,

Ranier Vilela

[1] https://www.postgresql.org/message-id/29478.1576537771%40sss.pgh.pa.us
[2] https://www.postgresql.org/message-id/30119.1576538578%40sss.pgh.pa.us