Bug 976398 - Check camera permissions directly instead of using Unix groups. r=mwu r=bent

Loosely based on the code used to check PRemoteOpenFile.
Bug 977859 - Drop uid 0 in all content processes immediately after fork. r=bent r=kang

Now all regular child processes, including preallocated, are deprivileged.
Only Nuwa needs uid 0, because each of its children has a different uid/gid.