Log Message:
Changes 1.1.1:
Use secure_getenv(3) to improve security

This patch is in response to the following security vulnerabilities
(CVEs) reported to NVIDIA against libvdpau:

CVE-2015-5198
CVE-2015-5199
CVE-2015-5200

To address these CVEs, this patch:

- replaces all uses of getenv(3) with secure_getenv(3);
- uses secure_getenv(3) when available, with a fallback option;
- protects VDPAU_DRIVER against directory traversal by checking for '/'

On platforms where secure_getenv(3) is not available, the C preprocessor
will print a warning at compile time. Then, a preprocessor macro will
replace secure_getenv(3) with our getenv_wrapper(), which utilizes the check:

  getuid() == geteuid() && getgid() == getegid()

See getuid(2) and getgid(2) for further details.