sm: implement connect

Add handles for peer's fifo and msgs, and pointer for mmap buffer.
When creating endpoint, store the mmap buffer's length in /path/msgs
so that client's can read the file to determine the length to mmap.
Add a parse uri function.
In connect, open their fifo, open their shared memory object,
get the mmap length, and mmap their buffer.
Check sep->conns for a duplicate ID before adding this conn.
Add cleanup.
sm: last commit failed to compile

Add minor fixes to make the compiler happy.