Construct a variable from name and value and put it in the environment while saving the previous value of the environment variable into a global list so it can be restored later. 
| const(char)[] name | the name of the variable | 
| const(char)[] value | the value of the variable | 
Allocate a new variable via xmalloc that can be added to the global environment. The resulting string will be null-terminated immediately after the end of the array.
| const(char)[] name | name of the variable | 
| const(char)[] value | value of the variable | 
Restore the original environment.
Save the environment variable name if not saved already.
    © 1999–2019 The D Language Foundation
Licensed under the Boost License 1.0.
    https://dlang.org/phobos/dmd_env.html