libstd/env: Add non-Rust synchronization warnings for setenv()
libstd/env: Add non-Rust synchronization warnings for setenv()

See:
https://sourceware.org/bugzilla/show_bug.cgi?id=4887#c9
https://bugs.freedesktop.org/show_bug.cgi?id=65681

I just noticed this while talking to someone who was using
`os.environ['FOO'] = 'BAR'` in Python and since I'm learning Rust, I
was curious if it did anything special here.  It looks like Rust has
an internal mutex, which helps for apps that are pure Rust, but it
will be an evil trap for someone later adding in native code (apps
like Servo and games will be at risk).

Java got this right by disallowing `setenv()` from the start.

I suggest Rust program authors only use `setenv()` early in main.

sae-bom/rust,cllns/rust,graydon/rust,dwillmer/rust,nwin/rust,reem/rust,mvdnes/rust,jashank/rust,pshc/rust,rohitjoshi/rust,zubron/rust,philyoon/rust,robertg/rust,KokaKiwi/rust,jroesch/rust,zubron/rust,zachwick/rust,pelmers/rust,seanrivera/rust,graydon/rust,zubron/rust,vhbit/rust,victorvde/rust,seanrivera/rust,jroesch/rust,zachwick/rust,omasanori/rust,kwantam/rust,omasanori/rust,gifnksm/rust,aepsil0n/rust,pshc/rust,XMPPwocky/rust,sae-bom/rust,reem/rust,zubron/rust,reem/rust,philyoon/rust,pshc/rust,ejjeong/rust,jashank/rust,aepsil0n/rust,andars/rust,GBGamer/rust,vhbit/rust,KokaKiwi/rust,zubron/rust,jroesch/rust,aidancully/rust,aneeshusa/rust,rohitjoshi/rust,aidancully/rust,jroesch/rust,GBGamer/rust,andars/rust,kwantam/rust,l0kod/rust,dwillmer/rust,KokaKiwi/rust,l0kod/rust,vhbit/rust,TheNeikos/rust,pelmers/rust,XMPPwocky/rust,rohitjoshi/rust,seanrivera/rust,mvdnes/rust,GBGamer/rust,aneeshusa/rust,rohitjoshi/rust,carols10cents/rust,omasanori/rust,mihneadb/rust,kwantam/rust,omasanori/rust,XMPPwocky/rust,mihneadb/rust,ejjeong/rust,graydon/rust,carols10cents/rust,krzysz00/rust,graydon/rust,kwantam/rust,robertg/rust,nwin/rust,ejjeong/rust,pelmers/rust,vhbit/rust,aneeshusa/rust,gifnksm/rust,ejjeong/rust,cllns/rust,dwillmer/rust,zachwick/rust,dwillmer/rust,l0kod/rust,nwin/rust,sae-bom/rust,aepsil0n/rust,mvdnes/rust,GBGamer/rust,carols10cents/rust,krzysz00/rust,jroesch/rust,pshc/rust,GBGamer/rust,krzysz00/rust,pelmers/rust,kwantam/rust,jashank/rust,pelmers/rust,dwillmer/rust,philyoon/rust,GBGamer/rust,XMPPwocky/rust,krzysz00/rust,mihneadb/rust,XMPPwocky/rust,philyoon/rust,omasanori/rust,carols10cents/rust,l0kod/rust,gifnksm/rust,l0kod/rust,mihneadb/rust,aneeshusa/rust,zachwick/rust,carols10cents/rust,jashank/rust,TheNeikos/rust,aidancully/rust,graydon/rust,aepsil0n/rust,andars/rust,jashank/rust,sae-bom/rust,l0kod/rust,vhbit/rust,krzysz00/rust,dwillmer/rust,reem/rust,victorvde/rust,andars/rust,ejjeong/rust,robertg/rust,zubron/rust,zubron/rust,graydon/rust,victorvde/rust,mvdnes/rust,zubron/rust,seanrivera/rust,GBGamer/rust,aneeshusa/rust,ejjeong/rust,TheNeikos/rust,nwin/rust,nwin/rust,cllns/rust,jashank/rust,pshc/rust,mvdnes/rust,gifnksm/rust,andars/rust,dwillmer/rust,jroesch/rust,aneeshusa/rust,nwin/rust,KokaKiwi/rust,jroesch/rust,philyoon/rust,XMPPwocky/rust,vhbit/rust,KokaKiwi/rust,mihneadb/rust,l0kod/rust,sae-bom/rust,pshc/rust,robertg/rust,pshc/rust,nwin/rust,robertg/rust,aepsil0n/rust,omasanori/rust,rohitjoshi/rust,TheNeikos/rust,seanrivera/rust,philyoon/rust,jroesch/rust,jashank/rust,zachwick/rust,jashank/rust,seanrivera/rust,vhbit/rust,cllns/rust,carols10cents/rust,rohitjoshi/rust,pelmers/rust,vhbit/rust,andars/rust,l0kod/rust,victorvde/rust,TheNeikos/rust,reem/rust,victorvde/rust,krzysz00/rust,nwin/rust,sae-bom/rust,cllns/rust,KokaKiwi/rust,robertg/rust,kwantam/rust,zachwick/rust,gifnksm/rust,pshc/rust,GBGamer/rust,aidancully/rust,gifnksm/rust,aepsil0n/rust,reem/rust,cllns/rust,mvdnes/rust,mihneadb/rust,TheNeikos/rust,aidancully/rust,dwillmer/rust,aidancully/rust,victorvde/rust