Adding the secure_getenv call to FreeBSD's libc

Dear FreeBSD Hackers,

I was wondering if adding the glibc extension call, `secure_getenv`, to 
FreeBSD's libc is allowed. Obviously, this would not only need to be 
permitted but also wanted. In that latter department, I could see the 
need arise for `secure_getenv` when porting applications written for 
Linux as they are often written targeting glibc. Also, this addition 
would bring us more inline with other libc implementations such as musl 
libc.

Thank you for your time,
Lucy