log: keep fallback syslog open between messages

[Komzpa]

On platforms that use the process-global syslog API, OpenSSH currently opens and closes syslog around each individual log message.

Keep that fallback syslog state open between OpenSSH log messages instead. log_init() still closes any OpenSSH-owned syslog state before changing ident/facility, and each log write still calls openlog() first so OpenSSH restores its expected ident/facility after any other library uses syslog.

The openlog_r() path is unchanged.

This reduces repeated fallback syslog open/close churn while preserving the lifecycle behavior that matters for reinitialization and foreign syslog users.

Regress coverage added for:

• keeping the fallback syslog state open across multiple OpenSSH log messages;
• closing/reopening on log_init() ident/facility changes;
• restoring OpenSSH ident/facility after foreign syslog use;
• bypassing syslog when a log handler is installed.

Tested:

• git diff --check
• ./regress/unittests/misc/test_misc
• make unit