vconsole: search for usable source console (#6180)

      When vconsole-setup is called without arguments, search for a usable
console instead of using /dev/tty0.

/dev/tty0 — pointing to the current active console — it not necessarily
usable and in such case vconsole-setup would exit with failure. In particular
when systemd-vconsole-setup.service was restarted from within an X
session, it always failed.

If the function searching for a usable source terminal fails, the first
encountered error is returned to the caller.

Closes #5367.

Additional changes:

- true/false functions with 'is_ prefix are renamed to functions with
  'verify_vc_' prefix and return 0 on success and negative error on
  failure
- O_NOCTTY flag is used when opening terminals