softraid: make verbose messages -n/nowrite aware

softraid: make verbose messages -n/nowrite aware

Using installboot(8) in dry-run mode aka. -vn (verbose + nowrite) and first
reading "would install" followed by an "installing" made me uneasy.

Luckily, nothing was written in dry-run mode, it's just that the
softraid specific code fails to honour -n/nowrite.

So make sure it does and pull the printf out of `if (!nowrite)' so we
see the "would install" messages in dry-run mode in the first place.

Output diff for root on CRYPTO softraid on biosboot(8/amd64) to illustrate:

    -# installboot -nv sd1
    +# ./obj/installboot -nv sd1
     would install bootstrap on /dev/rsd1c
     using first-stage /usr/mdec/biosboot, second-stage /usr/mdec/boot
     sd1: softraid volume with 1 disk(s)
    +sd1: would install boot loader on softraid volume
     /usr/mdec/boot is 6 blocks x 16384 bytes
    -sd0a: installing boot blocks on /dev/rsd0c, part offset 144
    +sd0a: would install boot blocks on /dev/rsd0c, part offset 144
     master boot record (MBR) at sector 0
     	partition 3: type 0xA6 offset 64 size 488392001
     /usr/mdec/biosboot will be written at sector 64

OK jsing