Allow display names to be arbitrary paths [v2]

This lets the local listening socket live in a protected directory
where other users cannot spoof it.

Signed-off-by: Keith Packard <keithp@keithp.com>

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v2:
	Update comment in VerifyDisplayName
4 jobs for listen-path in 10 minutes and 11 seconds (queued for 2 seconds)
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debian-buster

00:04:40

 
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autotools

00:05:30

passed #5738244
meson

00:02:06

passed #5738245
meson-noglamor

00:01:58