config: Allow overrides from the command line

This patch allows the configuration values to be overridden from the
command-line, with a new -config_overrides flag.

There is a fairly specific use case for this, when editing the
configuration file is not feasible or convenient (e.g. running an
user-supplied configuration in a managed environment).

docs/man/chasquid.1

@@ -133,7 +133,7 @@
 .\" ========================================================================
 .\"
 .IX Title "chasquid 1"
-.TH chasquid 1 "2018-07-22" "" ""
+.TH chasquid 1 "2020-05-16" "" ""
 .\" For nroff, turn off justification.  Always turn off hyphenation; it makes
 .\" way too many mistakes in technical documents.
 .if n .ad l
@@ -154,6 +154,11 @@ It's written in Go, and distributed under the Apache license 2.0.
 .IP "\fB\-config_dir\fR \fIdir\fR" 8
 .IX Item "-config_dir dir"
 configuration directory (default \fI/etc/chasquid\fR)
+.IP "\fB\-config_overrides\fR \fIconfig\fR" 8
+.IX Item "-config_overrides config"
+configuration values (in text protobuf format) to override the on-disk
+configuration with. This should only be needed in very specific cases for
+deployments where editing the configuration file is not feasible.
 .IP "\fB\-alsologtostderr\fR" 8
 .IX Item "-alsologtostderr"
 also log to stderr, in addition to the file

docs/man/chasquid.1.pod

@@ -22,6 +22,12 @@ It's written in Go, and distributed under the Apache license 2.0.
 
 configuration directory (default F</etc/chasquid>)
 
+=item B<-config_overrides> I<config>
+
+configuration values (in text protobuf format) to override the on-disk
+configuration with. This should only be needed in very specific cases for
+deployments where editing the configuration file is not feasible.
+
 =item B<-alsologtostderr>
 
 also log to stderr, in addition to the file