Opening libraries by soname

[Paolo Bonzini]

In debian, libncurses.so is not installed unless libncurses-dev is also 
installed.  Therefore, programs should open libncurses.so.5 directly.

However, this is not possible with ltdl_openext.  Would a patch be 
accepted to extend ltdl_openext("libncurses.5") like this:

- of course, opening "libncurses.5" tries to open libncurses.5.so and 
libncurses.5.la first

- on some systems it might succeed, as on Darwin.  on other it will 
fail, but it will recognize the soversion suffix and try to open 
libncurses.so.5.

- failing to do so, it will try libncurses.la and check the current/age 
items to detect if the library supports that soversion.

Paolo