allow hard-coded port /foo to be changed via YARP_RENAME_foo

Adds support for a port renaming method of last resort.
Suppose a program has a port called /foo/bar and there is no
way provided to change the name of that port other than
source code modification.  YARP already supports YARP_PORT_PREFIX
which allows us to add a prefix to all ports.  Now,
a port name can be changed entirely using YARP_RENAME_foo_bar.
For example:
  YARP_RENAME_read=/logger yarp read /read
will open a port named "/logger" for shells where this syntax
is permitted.  Renames (if present) are applied before prefixes
(if present).