init: check for filesystem where write bit cannot be removed

init: check for filesystem where write bit cannot be removed

This fixes a reversion caused by a99a84f3420a1d93e4ecd4b8cf30638b48e2758e,
when git-annex init is run as root on a FAT filesystem mounted with
hdiutil on OSX. Such a mount point has file mode 777 for everything and
it cannot be changed. The existing crippled filesystem test tried to
write to a file after removing write bit, but that test does not run as
root (since root can write to unwritable files). So added a check of the
write permissions of the file, after attempting to remove them.

Sponsored-by: Dartmouth College's Datalad project