Work around incorrect/broken pathnames in NT_FILE note

Work around incorrect/broken pathnames in NT_FILE note

Luis Machado reported some regressions after I pushed recent core file
related patches fixing BZ 25631:

    FAIL: gdb.base/corefile.exp: backtrace in corefile.exp
    FAIL: gdb.base/corefile.exp: core-file warning-free
    FAIL: gdb.base/corefile.exp: print func2::coremaker_local
    FAIL: gdb.base/corefile.exp: up in corefile.exp
    FAIL: gdb.base/corefile.exp: up in corefile.exp (reinit)

This commit fixes these regressions.  Thanks to Luis for testing
an earlier version of the patch.  (I was unable to reproduce these
regressions in various test environments that I created.)

Luis is testing in a docker container which is using the AUFS storage
driver.  It turns out that the kernel is placing docker host paths in
the NT_FILE note instead of paths within the container.

I've made a similar docker environment (though apparently not similar
enough to reproduce the regressions).  This is one of the paths that
I see mentioned in the warning messages printed while loading the
core file during NT_FILE note processing - note that I've shortened
the path component starting with "d07c4":

/var/lib/docker/aufs/diff/d07c4...21/lib/x86_64-linux-gnu/ld-2.27.so

This is a path on the docker host; it does not exist in the
container.  In the docker container, this is the path:

/lib/x86_64-linux-gnu/ld-2.27.so

My first thought was to disable all NT_FILE mappings when any path was
found to be bad.  This would have caused GDB to fall back to accessing
memory using the file stratum as it did before I added the NT_FILE
note loading code.  After further consideration, I realized that we
could do better than this.  For file-backed memory access, we can
still use the NT_FILE mappings when available, and then attempt to
access memory using the file stratum constrained to those address
ranges corresponding to the "broken" mappings.

In order to test it, I made some additions to corefile2.exp in which
the test case's executable is renamed.  The core file is then loaded;
due to the fact that the executable has been renamed, those mappings
will be unavailable.  After loading the core file, the executable is
renamed back to its original name at which point it is loaded using
GDB's "file" command.  The "interesting" tests are then run.  These
tests will print out values in file-backed memory regions along with
mmap'd regions placed within/over the file-backed regions.  Despite
the fact that the executable could not be found during the NT_FILE
note processing, these tests still work correctly due to the fact that
memory is available from the file stratum combined with the fact that
the broken NT_FILE mappings are used to prevent file-backed access
outside of the "broken" mappings.

gdb/ChangeLog:

	* corelow.c (unordered_set): Include.
	(class core_target): Add field 'm_core_unavailable_mappings'.
	(core_target::build_file_mappings): Print only one warning
	per inaccessible file.  Add unavailable/broken mappings
	to m_core_unavailable_mappings.
	(core_target::xfer_partial): Call...
	(core_target::xfer_memory_via_mappings): New method.

gdb/testsuite/ChangeLog:

	* gdb.base/corefile2.exp (renamed binfile): New tests.