fleetd: fix argument handling

[jonboulle]

We were calling userset.Usage but it was never actually defined. Unlike
in the top-level of the flag package (i.e. flag.Usage), this method
does not have a default value on FlagSets.

The only reason this never showed up way earlier in the ErrHelp path
(and was only revealed via 8ebb81b)
is that branch was never actually encountered.

Fixes #1360

[jonboulle]

: trevize fleet master ; bin/fleetd 
INFO fleetd.go:69: Starting fleetd version 0.11.5+git
INFO fleetd.go:175: No provided or default config file found - proceeding without
FATAL fleetd.go:97: Failed creating Server: mkdir /run/fleet: permission denied
: trevize fleet master ; bin/fleetd --version
fleetd version 0.11.5+git
: trevize fleet master ; bin/fleetd version
fleetd version 0.11.5+git
: trevize fleet master ; bin/fleetd foo
Usage of bin/fleetd:
  -config="": Path to config file. Fleet will look for a config at /etc/fleet/fleet.conf by default.
  -version=false: Print the version and exit
: trevize fleet master ; bin/fleetd --false
flag provided but not defined: -false
Usage of bin/fleetd:
  -config="": Path to config file. Fleet will look for a config at /etc/fleet/fleet.conf by default.
  -version=false: Print the version and exit
: trevize fleet master ; bin/fleetd asjkdlfkladsjfkladjfadslksdkl aljsdfldaks kjdkjkdkd
Usage of bin/fleetd:
  -config="": Path to config file. Fleet will look for a config at /etc/fleet/fleet.conf by default.
  -version=false: Print the version and exit
: trevize fleet master ; bin/fleetd --config/tmp/asdf
flag provided but not defined: -config/tmp/asdf
Usage of bin/fleetd:
  -config="": Path to config file. Fleet will look for a config at /etc/fleet/fleet.conf by default.
  -version=false: Print the version and exit
: trevize fleet master ; bin/fleetd --config=/tmp/asdf
INFO fleetd.go:69: Starting fleetd version 0.11.5+git
FATAL fleetd.go:162: Unable to use config file /tmp/asdf: stat /tmp/asdf: no such file or directory
: trevize fleet master ; bin/fleetd --help
Usage of bin/fleetd:
  -config="": Path to config file. Fleet will look for a config at /etc/fleet/fleet.conf by default.
  -version=false: Print the version and exit
: trevize fleet master ; bin/fleetd -h
Usage of bin/fleetd:
  -config="": Path to config file. Fleet will look for a config at /etc/fleet/fleet.conf by default.
  -version=false: Print the version and exit
: trevize fleet master ; bin/fleetd help
Usage of bin/fleetd:
  -config="": Path to config file. Fleet will look for a config at /etc/fleet/fleet.conf by default.
  -version=false: Print the version and exit
: trevize fleet master ; 

[mischief]

lgtm