Fix MNAMELEN or reimplement struct statfs

[Dirk Engling]

The arbitrary value

#define MNAMELEN        88              /* size of on/from name bufs */

struct statfs {
[...]
	char	f_mntfromname[MNAMELEN];/* mounted filesystem */
	char	f_mntonname[MNAMELEN];	/* directory on which mounted */
};

currently bites us when trying to use poudriere with errors like

'mount: tmpfs: File name too long'

/poudriere/data/build/91_RELEASE_amd64-REALLY-REALLY-LONG-JAILNAME/ref/wrkdirs

The topic has been discussed several times since 2004 and has been
postponed each time, the last time when it hit zfs users:

http://lists.freebsd.org/pipermail/freebsd-fs/2010-March/007974.html

So I'd like to point to the calendar, it's 2013 already and there's
still a static arbitrary (and way too low) limit in one of the core
areas of the vfs code.

So I'd like to bump the issue and propose either making f_mntfromname a
dynamic allocation or just increase MNAMELEN, using 10.0 as water shed.

Regards,

  erdgeist