Support for setting environment variables at run time (C#)

[tamas]

Certain features require to modify the environment setting for mapserver or the dependent libraries. In most cases these settings should be specified on a per process basis, a system wide setting would provide side effects for other applications using the same libraries.

Currently there's no chance to modify these settings for a mapcript c# application after the process has been started. This is because the getenv method is used to retrieve the environment parameter, but it operates only on the data structures accessible to the run-time library and not on the environment "segment" created for the process by the operating system. The most reasonable option to provide a solution would be to expose putenv to the mapscript C# interface.

[tamas]

The related changes are fairly simple with no harmful effects. Added to both the development version (r10807) and the stable branch (r10808).