env: avoid leaving dangling pointers in __env_map

This is the minimal fix for __putenv leaving a pointer to freed heap
storage in __env_map array, which could later on lead to errors such
as double-free.

include cleanups: remove unused headers and add feature test macros

as an extension, have putenv("VAR") behave as unsetenv("VAR")

the behavior of putenv is left undefined if the argument does not
contain an equal sign, but traditional implementations behave this way
and gnulib replaces putenv if it doesn't do this.

initial check-in, version 0.5.0