eliminate misleading flag parsing error
eliminate misleading flag parsing error

The standard mflags error reporting produces s.t. like
```
flag provided but not defined: -p
See '/home/weave/weaver --help'.
```
on stderr. This is misleading since the program name is container
local, so not something the user actually invoked.

Fixes #1321.

awh/weave,dpw/weave,dpw/weave,cgvarela/weave,narry/weave,weaveworks/weave,awanke/weave,squaremo/weave,paulbellamy/weave,aaam/weave,awh/weave,paulbellamy/weave,tomwilkie/weave,awanke/weave,tomwilkie/weave,rade/weave,aaam/weave,rade/weave,n054/weave,rade/weave,rade/weave,philipz/weave,squaremo/weave,n054/weave,weaveworks/weave,squaremo/weave,bboreham/weave,bboreham/weave,narry/weave,paulbellamy/weave,awh/weave,rade/weave,beni55/weave,beni55/weave,narry/weave,dpw/weave,tomwilkie/weave,tomwilkie/weave,beni55/weave,cgvarela/weave,narry/weave,squaremo/weave,aaam/weave,bboreham/weave,awh/weave,cgvarela/weave,beni55/weave,awanke/weave,squaremo/weave,philipz/weave,n054/weave,philipz/weave,dpw/weave,beni55/weave,paulbellamy/weave,cgvarela/weave,aaam/weave,bboreham/weave,bboreham/weave,awanke/weave,narry/weave,weaveworks/weave,tomwilkie/weave,awh/weave,n054/weave,dpw/weave,aaam/weave,philipz/weave,philipz/weave,weaveworks/weave,n054/weave,cgvarela/weave,awanke/weave