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