This is the port shared between all application processes which use it to pass
requests for processing.  Using it significantly simplifies the request
processing code in the router.  The drawback is 2 more file descriptors per each
configured application and more complex libunit message wait/read code.

The goal is to minimize the number of (pid, id) to port hash lookups which
require a library mutex lock.  The response port is found once per request,
while the read port is initialized at startup.

- Changed the port management callbacks to notifications, which e. g. avoids
the need to call the libunit function
- Added context and library instance reference counts for a safer resource
release
- Added the router main port initialization

An earlier attempt (ad6265786871) to resolve this condition on the
router's side added a new issue: the app could get a request before
acquiring a port.

Main process exiting before app process init may have caused hanging.

Each request processed in a separate goroutine.  In case of OOSM state,
during response write, request goroutine blocks on channel which waits
event from main thread about SHM_ACK message from router.

This patch includes packaging changes related to files move.