Check if realpath works on nonexistent files.

Check if realpath works on nonexistent files.

On some platforms the native realpath doesn't work with non-existent
files (this is actually specified in some versions of POSIX), however
the sftp spec says its realpath with "canonicalize any given path name".
On those platforms, use realpath from the compat library.

In addition, when compiling with -DFORTIFY_SOURCE, glibc redefines
the realpath symbol to the checked version, so redefine ours to
something else so we pick up the compat version we want.

bz#2428, ok djm@