Windows: Fixed warnings in `shell.hpp`.
Windows: Fixed warnings in `shell.hpp`.

The `spawn` functions return the `intptr_t`.  This patch deals with
warnings about implicitly casting this to `int`.

Review: https://reviews.apache.org/r/52065/

zmalik/mesos,craimbert/mesos,jpeach/mesos,neilconway/mesos,asamerh4/mesos,gsantovena/mesos,zmalik/mesos,jpeach/mesos,andschwa/mesos,verizonlabs/mesos,asamerh4/mesos,chhsia0/mesos,yuquanshan/customizedMesos,reneploetz/mesos,neilconway/mesos,zmalik/mesos,neilconway/mesos,gsantovena/mesos,verizonlabs/mesos,Aman-Jain-14/customizedMesos,yuquanshan/customizedMesos,reneploetz/mesos,verizonlabs/mesos,dforsyth/mesos,craimbert/mesos,neilconway/mesos,asamerh4/mesos,jpeach/mesos,asamerh4/mesos,gsantovena/mesos,asamerh4/mesos,Aman-Jain-14/customizedMesos,gsantovena/mesos,yuquanshan/customizedMesos,yuquanshan/customizedMesos,verizonlabs/mesos,craimbert/mesos,dforsyth/mesos,dforsyth/mesos,Aman-Jain-14/customizedMesos,jpeach/mesos,reneploetz/mesos,kaysoky/mesos,Aman-Jain-14/customizedMesos,Aman-Jain-14/customizedMesos,andschwa/mesos,abudnik/mesos,verizonlabs/mesos,asamerh4/mesos,Gilbert88/mesos,chhsia0/mesos,craimbert/mesos,reneploetz/mesos,zmalik/mesos,verizonlabs/mesos,Gilbert88/mesos,gsantovena/mesos,zmalik/mesos,craimbert/mesos,Gilbert88/mesos,kaysoky/mesos,gsantovena/mesos,reneploetz/mesos,yuquanshan/customizedMesos,chhsia0/mesos,Aman-Jain-14/customizedMesos,Aman-Jain-14/customizedMesos,craimbert/mesos,andschwa/mesos,asamerh4/mesos,kaysoky/mesos,gsantovena/mesos,andschwa/mesos,abudnik/mesos,kaysoky/mesos,yuquanshan/customizedMesos,verizonlabs/mesos,abudnik/mesos,jpeach/mesos,neilconway/mesos,abudnik/mesos,abudnik/mesos,neilconway/mesos,dforsyth/mesos,jpeach/mesos,chhsia0/mesos,chhsia0/mesos,Gilbert88/mesos,reneploetz/mesos,chhsia0/mesos,chhsia0/mesos,dforsyth/mesos,Gilbert88/mesos,abudnik/mesos,neilconway/mesos,andschwa/mesos,andschwa/mesos,kaysoky/mesos,jpeach/mesos,reneploetz/mesos,zmalik/mesos,Gilbert88/mesos,dforsyth/mesos,dforsyth/mesos,yuquanshan/customizedMesos,zmalik/mesos,andschwa/mesos,kaysoky/mesos,kaysoky/mesos,Gilbert88/mesos,abudnik/mesos,craimbert/mesos