flagString(flags, "default") should return default only if flags == 0.

Noticed by Han-Wen: https://review.gerrithub.io/c/hanwen/go-fuse/+/555490/comment/9cbd50ed_d836c29c/

Without the fix added test fails as

    --- FAIL: TestFlagStringDefault (0.00s)
        print_test.go:48: flagString(8, "X") -> got "X,0x8" ;  want "0x8"

Change-Id: I5b9432741e30948d490b276c097fbcfe45ecd936