mount: properly ignore comments in /etc/filesystems

mount: properly ignore comments in /etc/filesystems

The POSIX spec for sscanf() says that whitespace may be matched against 0
bytes which means doing sscanf(" %s") against "#foo" will result in a
match. You can see this behavior by using the verbose options on a garbage
file:

...
mount: you didn't specify a filesystem type for /dev/null
       I will try all types mentioned in /etc/filesystems or /proc/filesystems
Trying #
mount: mount(2) syscall: source: "/dev/null", target: "/", filesystemtype: "#", mountflags: -1058209792, data: (null)
Trying #vfat
mount: mount(2) syscall: source: "/dev/null", target: "/", filesystemtype: "#vfat", mountflags: -1058209792, data: (null)
...

Reported-by: Dave Barton <dave.barton@comodo.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>