poseven:  Add fd_pair::release( int i )

Release a single fd from the pair.
unet-connect:  Close the other control channel end

After forking the connector, close its end of the control channel in the
parent process.
unet-connect:  Only wait to reap connector on EOF

Only call waitpid() for the connector process if a read from the control
channel returns zero (which will happen if the process has exited.)  Any
future connectors that stay running will write an additional byte to the
control channel after passing the file descriptors, unblocking the read.
[NEW] uexec:  Connector that execs a chosen server
unix-tools:  Build uexec
archive.pl:  Ship uexec
Merge branch 'uexec'