fusefs: use fsname mount option if set

[ali.abdallah_suse.com]

Summary

Use fsname if it set as f_mntfromname, this improves the output of df -hT, which will become

/dev/da0s1 fusefs 58G 814M 57G 1% /media/extfat
instead of
/dev/fuse fusefs 58G 814M 57G 1% /media/extfat

This requires no changes to existing fuse-libs based filesystem or other fuse fs.

[ali.abdallah_suse.com]

Use vfs_getopt instead of vfs_getopts

[asomers]

Does anything already set fsname? libfuse, perhaps?

[ali.abdallah_suse.com]

In D35090#795234, @asomers wrote:

Does anything already set fsname? libfuse, perhaps?

Yes, libfuse calls mount_fusefs on FreeBSD with fsname set, expect that it is currently ignored by mount_fusefs.

Enabling it in mount_fusefs, and optionally set it as f_mntfromname if it exists in the fuse kernel driver works with no issue for me.

[ali.abdallah_suse.com]

Use specific m_flag for fsname= mount option field.

[asomers]

LGTM, and it works out of the box with at least one file system in ports.