core/loader: Don't call dlclose in a while loop

For awhile now, we've noticed continuous integration builds hanging on CentOS 6
64-bit build agents. After resolving a number of problems with symbols, strange
locks, and other shenanigans, the problem has persisted. In all cases, gdb
shows the Asterisk process stuck in loader.c on one of the infinite while loops
that calls dlclose repeatedly until success.

The documentation of dlclose states that it returns 0 on success; any other
value on error. It does not state that repeatedly calling it will eventually
clear those errors. Most likely, the repeated calls to dlclose was to force a
close by exhausting the references on the library; however, that will never
succeed if:
(a) There is some fundamental error at work in the loaded library that
    precludes unloading it
(b) Some other loaded module is referencing a symbol in the currently loaded
    module

This results in Asterisk sitting forever.

Since we have matching pairs of dlopen/dlclose, this patch opts to only call
dlclose once, and log out as an ERROR if dlclose fails to return success. If
nothing else, this might help to determine why on the CentOS 6 64-bit build agent
things are not closing successfully.

Review: https://reviewboard.asterisk.org/r/2970



git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@402287 f38db490-d61c-443f-a65b-d21fe96a405b