gh-131268: Adjust _PYTHREAD_NAME_MAXLEN on BSD

Adjust _PYTHREAD_NAME_MAXLEN constant on FreeBSD and NetBSD; define
the constant on OpenBSD. Initial patch by Xavier G.
(GitHub: @xavierog).

Author: vstinner

configure

@@ -7559,10 +7559,11 @@ _RESTORE_VAR([CPPFLAGS])
 case "$ac_sys_system" in
   Linux*) _PYTHREAD_NAME_MAXLEN=15;;  # Linux and Android
   SunOS*) _PYTHREAD_NAME_MAXLEN=31;;
-  NetBSD*) _PYTHREAD_NAME_MAXLEN=31;;
+  NetBSD*) _PYTHREAD_NAME_MAXLEN=15;; # gh-131268
   Darwin) _PYTHREAD_NAME_MAXLEN=63;;
   iOS) _PYTHREAD_NAME_MAXLEN=63;;
-  FreeBSD*) _PYTHREAD_NAME_MAXLEN=98;;
+  FreeBSD*) _PYTHREAD_NAME_MAXLEN=19;; # gh-131268
+  OpenBSD*) _PYTHREAD_NAME_MAXLEN=23;; # gh-131268
   *) _PYTHREAD_NAME_MAXLEN=;;
 esac
 if test -n "$_PYTHREAD_NAME_MAXLEN"; then