FreeBSD port (36): config support

[Bruno Haible]

> You have to decide whether you want to care about the FreeBSD sonames or not.
> I don't really know what the goals of your system are.

I want glibc to coexist with FreeBSD libc, assuming they are installed in
different directories. For the moment I assume the user will be responsible
for not mixing libraries of the two universes in the same directory, and
for using LD_LIBRARY_PATH with moderation.

> support FreeBSD-compatible soname
> libraries (if you want that) by having either the PT_INTERP value (i.e.
> the file name of the dynamic linker), or some magic the dynamic linker
> can detect, indicate disjoint library search paths.

Both native FreeBSD libraries and glibc-linked libraries have the same ELF
headers; the distinction will therefore come from the libc.so that a library
is linked with, like it was done to distinguish libc.so.5 and libc.so.6 on
Linux/{i386,m68k} a few years ago.

Bruno