dladdr support for dynamic linker (nonstandard extension)

dladdr support for dynamic linker (nonstandard extension)

based on patches submitted by boris brezillon. this commit also fixes
the issue whereby the main application and libc don't have the address
ranges of their mappings stored, which was theoretically a problem for
RTLD_NEXT support in dlsym; it didn't actually matter because libc
never calls dlsym, and it seemed to be doing the right thing (by
chance) for symbols in the main program as well.