rpc: limited multithread support for svc_nl

The rpc(3) itself was not designed with multithreading in mind, but we can
actually achieve some parallelism without modifying the library and the
framework. This transport will allow to process RPCs in threads, with
some hacks on the application side (documented in code). We make
reentrable only one method - SVC_REPLY(). Reading and parsing of incoming
calls is still done synchronously. But the actual processing of the calls
can be offloaded to a thread, and once finished the thread can safely
execute svc_sendreply() and the reply would be sent with the correct xid.

Differential Revision: https://reviews.freebsd.org/D48569