Make C-extension Ractor-safe

[mohamedhafez]

In planning ahead to the near future when TruffleRuby can run C-extensions marked with rb_ext_ractor_safe(true) in parallel, and for when Ractors are no longer just experimental, it would be great if the C-extension could be made Ractor-safe, or marked as such if it already is so! Would be happy to send in a PR in the later case.

[eregon]

There are some mutable global C variables in the extension:

syslog/ext/syslog/syslog.c

Lines 33 to 35 in f32e338

	static const char *syslog_ident = NULL;
	static int syslog_options = -1, syslog_facility = -1, syslog_mask = -1;
	static int syslog_opened = 0;

These are not thread-safe and most likely not Ractor-safe.
From what I see, the idea is to only allow a single opened Syslog instance at any one time.
That could be made thread-safe by using a mutex around the opening and closing and all these variables.