MFC r313448:

MFC r313448:

bsdinstall: Make sure chroot filesystems are umounted after use

* DISTDIR_IS_UNIONFS is set every time BSDINSTALL_DISTDIR is mounted inside
  BSDINSTALL_CHROOT. Use this flag to decide if it needs to be umounted
* BSDINSTALL_CHROOT/dev is mounted when 'bsdinstall mount' is called, there is
  no need to mount it again when user goes to shell after installation

Reviewed by:	allanjude
Obtained from:	pfSense
MFC after:	1 week
Sponsored by:	Rubicon Communications (Netgate)
Differential Revision:	https://reviews.freebsd.org/D8573

git-svn-id: svn://svn.freebsd.org/base/stable/10@313764 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f