ExtUtils::Install handle symbolic and hard links

[perl #72028]

When upgrading an already-installed file, ExtUtils::Install could mess up
the permissions of files if the old versions of files were hard or
symbolic links. For example, if the Foo module had been installed as
lib/Foo.pm and for some reason (perhaps due to OS packaging) that file was
hard-linked to other/Foo.pm or replaced with a symbolic link to
other/Foo.pm, then when trying to install a newer release of Foo, the
permissions of the other/Foo.pm file could end up messed up.

This was due to ExtUtils::Install changing the permissions of the old file
before unlinking it; if the file was a link, then the linked file would
get the chmod instead. Since on POSIXy platforms it is the directory
permissions, not the file permissions, that affect whether a file can be
unlinked, the chmod was redundant anyway. So on these platforms, skip the
chmod.

I've also added tests for symlinked and hard-linked files.