RFR: 8202353: os::readdir should use readdir instead of readdir_r

[Kim Barrett]

Please review this change to the various os::readdir implementations
for POSIX platforms to no longer use readdir_r, but instead use
readdir.  readdir_r has been deprecated by glibc and at least some BSD
variants, and seems to have been a mistake from the get-go.  See the
readdir_r Portability Note in recent versions of the glibc
documentation, e.g.
https://www.gnu.org/software/libc/manual/html_node/Reading_002fClosing-Directory.html.

Because we're no longer using readdir_r, the second argument for
os::readdir is now unused by any platform's implementation.  So we've
removed that argument, as well as the associated helper function
os::readdir_buf_size().  All callers have been updated to no longer
allocate and pass along the buffer for the second argument.

As part of this, merged all the (now identical) non-Windows
implementations of os::opendir, os::readdir, and os::closedir into
os_posix.cpp.  This means the os wrappers are no longer inlined, but
these aren't performance critical.

This also eliminated some "approximations" of the proper size for the
dirent argument that weren't correct (c.f. the deprecation rationale).
These places should have been using readdir_buf_size(), which would
have forced some of them to use C heap instead of stack allocation.

Eliminating the 2nd argument for os::readdir required fixing some uses
in jfr.  Some implementations of
SystemProcessInterface::SystemProcesses::ProcessIterator assumed the
second argument would always contain the same information as the
result, when the result is non-NULL.  But that was not part of the
os::readdir contract, and was never true for the Windows port (though
the Windows port uses a different mechanism for getting process info).
With JDK-8179887 it is no longer true for the Linux port either, which
broke some things in jfr; see JDK-8202835.  That bug is being fixed as
part of changing jfr for the new os::readdir signature, and the
relevant test is being reinstated.

Also updated various unqualified calls to opendir/closedir that were
actually calling the dirent.h functions directly to instead use the os
wrappers, for consistency.

Note that this is undoing the readdir-related parts of JDK-4647546.  

CR:
https://bugs.openjdk.java.net/browse/JDK-8202353
https://bugs.openjdk.java.net/browse/JDK-8202835

Webrev:
http://cr.openjdk.java.net/~kbarrett/8202353/open.00/

Testing:
mach5 tier1-5.
tier5 is where TestSystemProcess.java is run.
There are a number of jfr tests in tier3-4 that use process iterators on
relevant platforms.
There are tests related to PerfCounters scattered through tier1-3.