bsdinstall: Use TMPDIR if set

[freqlabs]

Summary

This allows separate bsdinstall workflows to run simultaneously without stomping on each other, by specifying different TMPDIR and BSDINSTALL_CHROOT paths for each installation. This is beneficial when using bsdinstall scripts to prepare several different VM volumes, for example.

If TMPDIR is unset, it defaults to /tmp, which preserves the existing behavior.

Sponsored by: iXsystems, Inc.

[freqlabs]

Use $BSDINSTALL_CHROOT in place of /mnt for umount.

[bcr]

Man page looks good to me.

[darkfiberiru_gmail.com]

Looks like a good change to me. Fixes my concern about doing parallel runs for things like vm's

[freqlabs]

@bcran Thanks! Would you mind committing this for me? I don't have the bit.

[bcran]

In D22979#504080, @ryan_freqlabs.com wrote:

@bcran Thanks! Would you mind committing this for me? I don't have the bit.

Sure! I'll try and get it committed today.

[freqlabs]

In D22979#505271, @bcran wrote:

Sure! I'll try and get it committed today.

Thank you! :)

[bcran]

In D22979#507973, @ryan_freqlabs.com wrote:

Thank you! :)

Sorry for the delay, I've been ill.

[bcran]

Fixed in r356740 (https://svnweb.freebsd.org/base?view=revision&revision=356740).