Improve names for function arguments in <unistd.h> for Studio.

Spotted while adding syncfs(2). I'd previously seen that this file already
had names, but apparently not inspected them. A couple were missing, a
couple (pipe/pipe2) could usefully have a more specific type, and many
hadruntogethercstylenames.

Bug: http://b/64613623
Test: builds
Change-Id: I6b4a131bf51bf04825317ec481bc55e87c88c2b3