Linux: Only send a control message if necessary
Linux: Only send a control message if necessary

Don't send a control message if we have no actual auxiliary data
(channels / shared memory regions) to transfer.

This shaves off another 6 or 7 per cent from small transfers (without
FDs) on my system.

test size_00_1    ... bench:       2,154 ns/iter (+/- 33)
test size_01_2    ... bench:       2,203 ns/iter (+/- 42)
test size_02_4    ... bench:       2,231 ns/iter (+/- 51)
test size_03_8    ... bench:       2,231 ns/iter (+/- 28)
test size_04_16   ... bench:       2,290 ns/iter (+/- 47)
test size_05_32   ... bench:       2,261 ns/iter (+/- 57)
test size_06_64   ... bench:       2,316 ns/iter (+/- 51)
test size_07_128  ... bench:       2,247 ns/iter (+/- 38)
test size_08_256  ... bench:       2,266 ns/iter (+/- 45)
test size_09_512  ... bench:       2,572 ns/iter (+/- 52)
test size_10_1k   ... bench:       2,488 ns/iter (+/- 45)
test size_11_2k   ... bench:       3,791 ns/iter (+/- 51)
test size_12_4k   ... bench:       5,365 ns/iter (+/- 54)
test size_13_8k   ... bench:       9,235 ns/iter (+/- 84)
test size_14_16k  ... bench:       8,833 ns/iter (+/- 102)
test size_15_32k  ... bench:      13,370 ns/iter (+/- 117)
test size_16_64k  ... bench:      22,004 ns/iter (+/- 717)
test size_17_128k ... bench:      43,369 ns/iter (+/- 976)
test size_18_256k ... bench:     224,096 ns/iter (+/- 75,219)
test size_19_512k ... bench:     458,353 ns/iter (+/- 149,531)
test size_20_1m   ... bench:   1,357,956 ns/iter (+/- 187,198)
test size_21_2m   ... bench:   1,781,991 ns/iter (+/- 512,027)
test size_22_4m   ... bench:   4,940,065 ns/iter (+/- 1,099,861)
test size_23_8m   ... bench:   9,345,216 ns/iter (+/- 1,557,181)

test size_00_1    ... bench:     222,064 ns/iter (+/- 8,292)
test size_01_2    ... bench:     224,589 ns/iter (+/- 4,033)
test size_02_4    ... bench:     226,667 ns/iter (+/- 4,774)
test size_03_8    ... bench:     229,002 ns/iter (+/- 5,107)
test size_04_16   ... bench:     224,895 ns/iter (+/- 3,323)
test size_05_32   ... bench:     230,973 ns/iter (+/- 3,265)
test size_06_64   ... bench:     224,377 ns/iter (+/- 5,778)
test size_07_128  ... bench:     229,364 ns/iter (+/- 8,282)
test size_08_256  ... bench:     235,654 ns/iter (+/- 3,860)
test size_09_512  ... bench:     235,874 ns/iter (+/- 6,021)
test size_10_1k   ... bench:     246,200 ns/iter (+/- 2,626)
test size_11_2k   ... bench:     386,233 ns/iter (+/- 5,313)
test size_12_4k   ... bench:     542,364 ns/iter (+/- 8,671)
test size_13_8k   ... bench:     929,892 ns/iter (+/- 12,989)
test size_14_16k  ... bench:     868,966 ns/iter (+/- 7,768)
test size_15_32k  ... bench:   1,342,400 ns/iter (+/- 6,781)
test size_16_64k  ... bench:   2,202,955 ns/iter (+/- 86,042)
test size_17_128k ... bench:   4,323,643 ns/iter (+/- 41,162)
test size_18_256k ... bench:   8,708,255 ns/iter (+/- 2,162,868)
test size_19_512k ... bench:  19,281,153 ns/iter (+/- 5,449,494)
test size_20_1m   ... bench: 102,875,141 ns/iter (+/- 10,584,288)
test size_21_2m   ... bench: 134,820,099 ns/iter (+/- 13,015,545)
test size_22_4m   ... bench: 446,291,502 ns/iter (+/- 12,073,997)
test size_23_8m   ... bench: 876,656,087 ns/iter (+/- 16,439,303)

servo/ipc-channel