Wire protocol user agent

As of v25 a user agent string is sent in the wire protocol's introduction packet INTR.

This user agent is required for v25+ clients.

A skycoin user agent has the following format:

$NAME:$VERSION($REMARK)

$NAME and $VERSION are required.

• $NAME is the coin or application's name, e.g. Skycoin. It can contain the following characters: A-Za-z0-9\-_+.
• $VERSION must be a valid semver version, e.g. 1.2.3 or 1.2.3-rc1. Semver has the option of including build metadata such as the git commit hash, but this is not included by the default client.
• $REMARK is optional. If not present, the enclosing brackets () should be omitted. It can contain the following characters: A-Za-z0-9\-_+;:!$%,.=?~ (includes the space character).

The v0.25.0 skycoin core implementation sends the following user agent: skycoin:0.25.0

An additional remark can be specified by the user with --user-agent-remark:

./run-client.sh --user-agent-remark=hello

The user agent will be skycoin:0.25.0(hello).