They're in glibc, though not in musl.
Also add basic doc comments to the whole of <sys/uio.h>.
Bug: http://b/203002492
Test: treehugger
Change-Id: Ic607f7f349e5b7c9bf66c25b7bd68f827da530d6
LLDB uses process_vm_readv quite extensively (it's an order of
magnitude faster than PTRACE_PEEKDATA). Add a bit more tests of
the function to make sure it stays working.
Change-Id: I64e17e6d56842f118a9485e3a18f42ca5e1b7577
Test: run the new test
