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