[stdio] Use FLOCKing in std{in,out,err}

Upstream musl lets some FILEs used internally bypass locking, which is
safe as they are not shared. It also did not enable FILE locking for
single threaded programs. We removed those code paths, but I
accidentally also missed resetting these locks.

MG-363 #done

Change-Id: I54f3a62d1c5b9ea8e8249b1e19c1ba33bd688072

[ulib][musl] Reduce unnecessary exports

There are still lots of symbols in the export list that should not
really be part of the ABI. But this picks off some low-hanging fruit.

Change-Id: I86168858de89bd327fb70cd4ca639caa1e502702

[magenta] Initial commit