MFC r329726, r329737, r329740

r329726:
Honor settings for including / excluding cd9660, ufs, ext2fs and msdos.

The Makefile gives the impression that ext2fs and msdos were excluded
(they weren't) and that you could exclude cd9660 and ufs support (you
couldn't). Allow those to be excluded.

We need to look, in the future, at trimming the number of supported
filesystems, and this will make that easier.

r329737:
Purely whitespace changes bringing this file closer to style(9).

Curiously, changing whitespace seems to cause the md5 of the .o files to differ
these days hence the following testing strategy:

Tested by: objdump -d | md5 (both in-tree clang and lang/gcc6)

r329740:
Further style(9) changes.

Tested by: objdump -d | md5 (both in-tree clang and lang/gcc6)