Don't recognize a file as an a.out shared library unless it has at
least 2 version numbers. This fixes the bug where the dynamic
linker would try to load an ELF shared library if it found one.

Note, this change also fixes the same thing in "ld", because the
code is shared.

For "ld" there is still a problem with ".a" libraries, which cannot
be distinguished by name. I haven't decided what, if anything, to
do about that.