Added generic support for loadable Squid modules or plugins.

Added generic support for loadable Squid modules or plugins.

The "loadable_modules" squid.conf option specifies what shared libraries to
load dynamically. The support is based on libtool's ltdl library, which has
been integrated with Squid prior to this commit, along with the
--enable-loadable-modules ./configure option.

Loaded modules are currently never unloaded, but that may change.

Loadable modules are needed for eCAP, but loadable_modules code deals with
generic module manipulation, independent from eCAP support.

TODO: support cachemgr reporting and reconfiguration.