cmd/gomobile: fix subcommand help message handling.
cmd/gomobile: fix subcommand help message handling.

1) add subcommand in the usage printout.
2) exit after printing out usage during flag parsing.

Change-Id: I57da849eca3c297f67437bdee1d36e5d340c2a58
Reviewed-on: https://go-review.googlesource.com/4836
Reviewed-by: David Crawshaw <a2f0bf29257797dd66800b4fbb9945c3e216c004@golang.org>

MaxCy330/mobile,steeve/mobile,0x90sled/mobile,goldcaddy77/mobile,MaxCy330/mobile,DanielWorld/mobile,xing/gomobile,swisspol/mobile,ichu501/mobile,swisspol/mobile,ppamorim/mobile,DanielWorld/mobile,golang/mobile,lonphy23t/mobile,ichu501/mobile,goldcaddy77/mobile,qskycolor/mobile,swisspol/mobile,lonphy23t/mobile,tempbottle/mobile,qskycolor/mobile,b54898533/mobile,tempbottle/mobile,ppamorim/mobile,lonphy23t/mobile,samanalysis/mobile,cainiaocome/mobile,ichu501/mobile,zach-klippenstein/mobile,xing/gomobile,steeve/mobile,b54898533/mobile,MaxCy330/mobile,samanalysis/mobile,MaxCy330/mobile,0x90sled/mobile,swisspol/mobile,ichu501/mobile,lonphy23t/mobile,golang/mobile,samanalysis/mobile,tempbottle/mobile,samanalysis/mobile,samanalysis/mobile,0x90sled/mobile,0x90sled/mobile,goldcaddy77/mobile,steeve/mobile,ppamorim/mobile,cainiaocome/mobile,0x90sled/mobile,cainiaocome/mobile,golang/mobile,DanielWorld/mobile,lonphy23t/mobile,freeman0211/mobile,DanielWorld/mobile,MaxCy330/mobile,tempbottle/mobile,goldcaddy77/mobile,b54898533/mobile,cainiaocome/mobile,swisspol/mobile,freeman0211/mobile,xing/gomobile,xing/gomobile,tempbottle/mobile,cainiaocome/mobile,xing/gomobile,zach-klippenstein/mobile,golang/mobile,ichu501/mobile,b54898533/mobile,ppamorim/mobile,freeman0211/mobile,b54898533/mobile,steeve/mobile,zach-klippenstein/mobile,freeman0211/mobile,zach-klippenstein/mobile,qskycolor/mobile,ppamorim/mobile,goldcaddy77/mobile,qskycolor/mobile