[lldb] Prevent 'process connect' from using local-only plugins

Add a 'can_connect' parameter to Process plugin initialization, and use
it to filter plugins to these capable of remote connections.  This is
used to prevent 'process connect' from picking up a plugin that can only
be used locally, e.g. the legacy FreeBSD plugin.

Differential Revision: https://reviews.llvm.org/D91810