Rewrite build scripts

New script build.sh is much more suitable for downstream packaging
(e.g. ./build.sh package) than hacked together package.sh wrapper for
get.sh while still being usable for "effort-less" installation.

Additionally, hostname setting in get.sh is flawed in many ways and is
not reimplemented in build.sh.

build.sh has proper command line options that allow to customize build
configuration and installation prefixes.
Documentation page get.sh is removed since all applicable environment
variables and flags are documented in ./build.sh --help.

build.sh can be called from the source directory to build maddy from
*this* source instead of forced 'go get' that was used in get.sh.
However, if build.sh is called not from the source directory, it clones
the repo and (optionally) uses the specified commit. This keeps build.sh
usable in curl|bash commands.

Due to the way source code is fetched, build.sh uses Git tags instead of
Go module versions as get.sh did.

docs/tutorials/setting-up.md

@@ -9,7 +9,7 @@ For purposes of clarity, these values are used in this tutorial as examples,
 wherever you see them, you need to replace them with your actual values:
 
 - Domain: example.org
-- MX domain (hostname): example.org
+- MX domain (hostname): mx.example.org
 - IPv4 address: 10.2.3.4
 - IPv6 address: 2001:beef::1
 
@@ -36,19 +36,27 @@ distributions, this should be enough:
 # apt-get install build-essential
 ```
 
-get.sh script will do the rest for you:
+build.sh script will do the rest for you:
 
 ```
-$ curl 'https://foxcpp.dev/maddy/get.sh' | bash
+$ curl 'https://foxcpp.dev/maddy/build.sh' | bash
 ```
 
-It will leave `maddy-setup` directory lying around, you might want to keep it
-so you don't have to redownload and recompile everything on update.
-
 *Note:* If you can't / don't use this script for some reason, instructions for
 manual installation can be found
 [here](../manual-installation)
 
+## Host name + domain
+
+Open /etc/maddy/maddy.conf with ~~vim~~your favorite editor and change
+the following lines to match your server name and domain you want to handle
+mail for.
+
+```
+$(hostname) = mx.example.org
+$(primary_domain) = example.org
+```
+
 ## TLS certificates
 
 One thing that can't be automagically configured is TLS certs. If you already
@@ -168,7 +176,7 @@ For Debian-based distributions:
 apt-get install fail2ban
 ```
 
-2. get.sh already installed necessary jail configuration files, but you have to
+2. build.sh already installed necessary jail configuration files, but you have to
    enable them. Open /etc/fail2ban/jail.d/common.local (create one if needed)
    and add the following lines:
 ```