mandoc: Add an .St argument for C23.

mandoc: Add an .St argument for C23.

[schwarze]

Log Message:
-----------
Add an .St argument for C23.

The mismatch of the year numbers ist not a typo.
The official name is "ISO/IEC 9899:2024", so the "4" is correct there.

But as a colloquial name, "C23" is more widespread than "C24",
probably because __STDC_VERSION__ == 202311L: The final version
of the standard was ready in 2023, only formally publishing it
took about 11 months after it was ready.

The main reason for choosing "-isoC-2023" as the .St argument
is to match what NetBSD already committed to their tree.
Also, it aligns well with the existing "-isoC-2011".

Based on an incorrect patch from Thomas Klausner <wiz at netbsd dot org>,
fixed by me.

OK jmc@

Modified Files:
--------------
    mandoc:
        st.c

Revision Data
-------------
Index: st.c
===================================================================
RCS file: /home/cvs/mandoc/mandoc/st.c,v
diff -Lst.c -Lst.c -u -p -r1.18 -r1.19
--- st.c
+++ st.c
@@ -54,6 +54,7 @@ LINE("-isoC-tcor1",	"ISO/IEC 9899/TCOR1:
 LINE("-isoC-tcor2",	"ISO/IEC 9899/TCOR2:1995 (\\(lqISO\\~C90, Technical Corrigendum 2\\(rq)")
 LINE("-isoC-99",	"ISO/IEC 9899:1999 (\\(lqISO\\~C99\\(rq)")
 LINE("-isoC-2011",	"ISO/IEC 9899:2011 (\\(lqISO\\~C11\\(rq)")
+LINE("-isoC-2023",	"ISO/IEC 9899:2024 (\\(lqISO\\~C23\\(rq)")
 LINE("-iso9945-1-90",	"ISO/IEC 9945-1:1990 (\\(lqPOSIX.1\\(rq)")
 LINE("-iso9945-1-96",	"ISO/IEC 9945-1:1996 (\\(lqPOSIX.1\\(rq)")
 LINE("-iso9945-2-93",	"ISO/IEC 9945-2:1993 (\\(lqPOSIX.2\\(rq)")
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