Make fastboot command-line parsing a bit more like adb.

Only show all the help if asked to, and have a few more descriptive
syntax errors.

Also show the help on stdout rather than stderr.

Bug: N/A
Test: manually ran "fastboot flash"/"fastboot update"/"fastboot flashall"
Change-Id: I59abd60e58a56fe7e44da5116a702087c36e14ce