pkg/osutil: add FilesExist/CopyFiles/LinkFiles functions
pkg/osutil: add FilesExist/CopyFiles/LinkFiles functions

Will be required by the new build system.

hyz0906/syzkaller,xairy/syzkaller,hyz0906/syzkaller,zoulasc/syzkaller,google/syzkaller,hyz0906/syzkaller,xairy/syzkaller,dotweiba/syzkaller,dotweiba/syzkaller,dotweiba/syzkaller,xairy/syzkaller,hyz0906/syzkaller,dotweiba/syzkaller,google/syzkaller,google/syzkaller,zoulasc/syzkaller,xairy/syzkaller,xairy/syzkaller,xairy/syzkaller,zoulasc/syzkaller,google/syzkaller,hyz0906/syzkaller,google/syzkaller,xairy/syzkaller,google/syzkaller,zoulasc/syzkaller,google/syzkaller