MFC r268628, r268631, r268635:

Convert *rootDir from external to static.

fsck_msdosfs: be a bit more permissive.

The free space value in the FSInfo block is merely unitialized when it is
0xffffffff. This fixes a bug found in NetBSD.

Respect FSFIXFAT.

Fix some whitespace issues while here.

Obtained from: NetBSD (CVS rev. 1.22, rev. 1.9), OpenBSD (misc)