stand: Add print_delay environment variable in loader

This adds support for a new print_delay environment variable,
which inserts a delay in microseconds when putchar encounters a
newline character. This can be useful when debugging.

Reviewed by: markj, imp, ziaee, mckusick (mentor)
Approved by: markj, imp, ziaee, mckusick (mentor)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D50832