BUG/MINOR: configure: pick the first valid UNIX socket from HAPROXY_MASTER_CLI

[fabiomatavelli]

Fixes #329. This picks up the work started in #330, which was closed by its author, and addresses the review feedback left there.

Problem

Since haproxy/haproxy@8a02257, HAProxy advertises its master CLI as a ;-separated list of addresses, so HAPROXY_MASTER_CLI now looks like:

unix@/data/haproxy/run/master-socket;sockpair@7

configureAPI only stripped the unix@ prefix and passed the rest through, so the runtime client tried to dial a socket literally named /data/haproxy/run/master-socket;sockpair@7:

level=warning msg="Error setting up runtime client with master socket (1): /data/haproxy/run/master-socket;sockpair@7 : dial unix /data/haproxy/run/master-socket;sockpair@7: connect: no such file or directory"

When the Data Plane API runs from a program section this makes it exit, and HAProxy tears down the whole process tree through exit-on-failure.

Changes

misc.MasterSocketFromEnv(value string) (string, bool) parses the raw environment value:

• splits on ; and keeps only the UNIX addresses, so any other address family HAProxy may add later is ignored instead of breaking the parsing
• returns the first candidate that is already bound on the filesystem (os.Stat plus os.ModeSocket)
• if none is bound yet, returns the first syntactically valid candidate anyway. runtime_options.AllowDelayedStart exists precisely to wait for a socket HAProxy has not bound yet, so a hard existence check here would regress those setups
• returns false when there is no usable UNIX socket at all

misc.IsUnixSocketAddr now rejects the empty string, and accepts an <family>@ prefix only when the family is unix. Previously "", sockpair@7 and fd@3 all returned true.

configureAPI only overrides haproxyOptions.MasterRuntime when a socket was actually found, and logs a warning otherwise. This is what keeps a master runtime set through --master-runtime / haproxy.master_runtime from being silently replaced by an empty string.

Tests: table-driven unit tests for both functions in misc/misc_test.go, binding real UNIX sockets so the os.ModeSocket branch is genuinely exercised.

Review comments from #330

Comment	Addressed by
@mjuraga: HAPROXY_MASTER_CLI is ;-separated and may hold several entries, split it and use the first valid unix@ one	MasterSocketFromEnv splits and iterates, first bound socket wins
@georgijd-form3: IsUnixSocketAddr("") is true, so an unset variable wipes a user-supplied MasterRuntime	IsUnixSocketAddr rejects "", and the caller only assigns when a socket was found
@mjuraga: IsUnixSocketAddr should validate the address properly, including that the socket exists	Address family validation lives in IsUnixSocketAddr, the existence check lives in MasterSocketFromEnv where a filesystem lookup is meaningful and where a delayed start can still be honoured

Notes

• abns@name now returns false from IsUnixSocketAddr. The runtime client cannot dial abstract-namespace sockets anyway, so this turns a confusing dial failure into an explicit skip.
• Verified locally: go build ./..., go test ./..., make lint (0 issues) and make check-commit all pass.