main: cut down "flag provided but not defined" error message

Don't dump the help text on the unsuspecting user, but
give a short error message:

  $ gocryptfs -foobar
  flag provided but not defined: -foobar
  Invalid command line: gocryptfs -foobar. Try 'gocryptfs -help'.

For comparison: This is what cp does:

  $ cp --foo
  cp: unrecognized option '--foo'
  Try 'cp --help' for more information.

And this what we used to do:

  $ gocryptfs -foobar
  flag provided but not defined: -foobar
  gocryptfs v1.4.4-45-gfb772da; go-fuse v20170619-35-gb16719c; 2018-06-08 go1.10.2

  Usage: gocryptfs -init|-passwd|-info [OPTIONS] CIPHERDIR
    or   gocryptfs [OPTIONS] CIPHERDIR MOUNTPOINT

  Common Options (use -hh to show all):
    -aessiv            Use AES-SIV encryption (with -init)
    -allow_other       Allow other users to access the mount
    -config            Custom path to config file
    -ctlsock           Create control socket at location
    -extpass           Call external program to prompt for the password
    -fg                Stay in the foreground
    -fusedebug         Debug FUSE calls
    -h, -help          This short help text
    -hh                Long help text with all options
    -init              Initialize encrypted directory
    -info              Display information about encrypted directory
    -masterkey         Mount with explicit master key instead of password
    -nonempty          Allow mounting over non-empty directory
    -nosyslog          Do not redirect log messages to syslog
    -passfile          Read password from file
    -passwd            Change password
    -plaintextnames    Do not encrypt file names (with -init)
    -q, -quiet         Silence informational messages
    -reverse           Enable reverse mode
    -ro                Mount read-only
    -speed             Run crypto speed test
    -version           Print version information
    --                 Stop option parsing
  You passed: "-foobar"
  flag provided but not defined: -foobar

0 files changed, 0 insertions, 0 deletions