MFC r284780, r284811, r284812:

MFC r284780, r284811, r284812:

 r284780:
  Ensure ASSUME_ALWAYS_YES is set when deleting pkg(8) from the
  DESTDIR, otherwise it can get stuck in ttyin.

 r284811:
  Consider PRERELEASE builds snapshots, in addition to CURRENT
  and STABLE.

 r284812:
  Change the hour:minute delimiter from ':' to '-', since
  the former is an invalid character in EC2 images.

Approved by:	re (glebius, insta-MFC)
Sponsored by:	The FreeBSD Foundation

git-svn-id: svn://svn.freebsd.org/base/stable/10@284813 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f