StartupInfoBuilder is a helper for constructing StartupInfoEx structures
featuring proc/thread attribute lists. Calling its setters triggers the
appropriate setting of fields, adjusting flags as necessary, and populating
the proc/thread attribute list as necessary. Currently it supports four
features: setting std handles, setting pseudo-consoles, specifying handles
for inheritance, and specifying jobs.
The conpty package simplifies creation of pseudo-consoles, their associated
pipes, and assignment of the pty to StartupInfoEx proc/thread attributes.
Updates #12383
Signed-off-by: Aaron Klotz <aaron@tailscale.com>