improve quickstart and readme for better first-time experience

- make the example commands in the readme more likely to succeed, especially
  for people who are not familiar with go and its toolchain.
- improve probability that the correct configuration is generated, especially
  the hostname. previously, if the quickstart email address was "some.domain",
  and the machine where you ran the quickstart was "myhost", the hostname used
  for the configuration was assumed to be "myhost.some.domain". but this is often
  not correct, especially when configuring mox to serve mail on a subdomain of an
  existing domain. mox will now try to determine the host name by a reverse
  lookup of the public ips it found. and it will warn if there are no/multiple
  candidates.

based on feedback from erik dubbelboer, thanks!

README.md

@@ -67,11 +67,21 @@ Mox was created by Mechiel Lukkien, mechiel@ueber.net.
 
 # Download
 
-You can easily (cross) compile mox if you have a Go toolchain installed:
+You can easily (cross) compile mox if you have a recent Go toolchain installed
+(see "go version", it must be >= 1.19; otherwise, see https://go.dev/dl/ or
+https://go.dev/doc/manage-install and $HOME/go/bin):
 
-	go install github.com/mjl-/mox@latest
+	GOBIN=$PWD go install github.com/mjl-/mox@latest
 
-Or you can download binaries from https://beta.gobuilds.org/github.com/mjl-/mox
+Or you can download a binary built with the latest Go toolchain from
+https://beta.gobuilds.org/github.com/mjl-/mox, and symlink or rename it to
+"mox".
+
+Verify you have a working mox binary:
+
+	./mox version
+
+Note: Mox only compiles/works on unix systems, not on Plan 9 or Windows.
 
 
 # Quickstart