ldso: dlopen could allocate a handle that was already in use

[navytux]

Currently dlopen allocates DSO handles by iterating loaded libraries
while counting them from 1. This would work if libraries would be never
unloaded, but it breaks in the presense of dlcose, e.g.:

dlopen('A') -> 1
dlopen('B') -> 2
dlopen('C') -> 3
dlclose('A') ; 1 is deleted from loadedLibs
dlopen('D') : should be either 1 or 4 but it gives 3, thus erroneously overwriting C.

Fix it by always using simply incrementing integer for next allocated
handle.

Suggested-by: @sbc100

[navytux]

( spawned from #7512 )