Fix exec -o bug

Fix bug where if there were multiple extensions to file (.tar.gz) we
would leave out periods when reassembling extension. Also default to
.txt extension is no extension is given.
Add remote signals

In client use select() to multiplex the proc's stdout/err and the
socket so we can simultaneously line buffer the proc output to the
server and listen for anything coming from the server (remote
signal).

Fixes #34