Implement first client command

The client can now talk to the old Perl signer implementation. Running
socat has been documented in README.md. Commonly used I/O code has been
moved to the shared/io.go file.

README.md

@@ -0,0 +1,26 @@
+# CAcert signer in Go
+
+This is a reimplementation of the CAcert signer code from the
+[CommModule directory](https://git.cacert.org/gitweb/?p=cacert-devel.git;a=tree;f=CommModule) of the CAcert software
+in [Go](https://golang.org/). The goal of this effort is to provide a more maintainable version of the software.
+
+## Running the signer and client locally
+
+The signer is usually attached to a USB serial port. You can run the signer and client locally using the
+[socat](https://manpages.debian.org/buster/socat/socat.1.en.html) utility.
+
+```shell script
+socat -d -d PTY,link=$(pwd)/ttyS0 PTY,link=$(pwd)/ttyS1
+```
+
+You may then run the server
+
+```shell script
+go run signer/main.go -a $(pwd)/dev/ttyS0
+```
+
+and the client:
+
+```shell script
+go run client/main.go -a $(pwd)/dev/ttyS1
+```