server: read/write now goes through connection fn's

server: read/write now goes through connection fn's

depending on whether the connection is over a socket
or pipe, the read is done differently.

pipes can return -1 when writing 0 bytes, make 0 byte
writes a successful no-op. 0 byte writes falls out
naturally of tcl server code.

Signed-off-by: Øyvind Harboe <oyvind.harboe@zylin.com>