chasquid-util: Remove dependency on docopt-go

The docopt-go library is quite convenient, but it has been abandoned for
a while :(

Since we only use it for chasquid-util, this patch removes it and
replaces it with a custom small parser, that is a reasonable fit for the
required use cases.

The patch also adds a couple of tests to increase coverage.

NOTE: docopt-go accepted some undocumented behaviour, in particular the
use of "-a b" instead of "-a=b". The new parser does not, so some
user scripts may require updating.

I think this should be rare enough not to be worth the complexity of
adjusting the parser to allow it.

docs/relnotes.md

@@ -5,6 +5,14 @@ This file contains notes for each release, summarizing changes and explicitly
 noting backward-incompatible changes or known security issues.
 
 
+## 1.7 (TODO)
+
+- chasquid-util no longer depends on the unmaintained docopt-go.
+  If you relied on undocumented parsing behaviour before, your invocations may
+  need adjustment.  In particular, `--a b` is no longer supported, and `--a=b`
+  must be used instead.
+
+
 ## 1.6 (2020-11-22)
 
 - Pass the EHLO domain to the post-data hook.