Set default pthread stack size to 256 KB

This patch tries to fix various crashes for applications depending on libmilter
by setting the stack size for pthreads to 256 KB. The default stack size for
musl libc is set to 80 KB whereas glibc has it set to 8 MB. This causes problems
when a large amount of memory is allocated on the stack.

For example, opendkim allocates blocks of 64 KB multiple times, which causes
libmilter (and therefore opendkim) to crash. For now, a stack size of 256 KB
looks sufficient and makes opendkim stop crashing.

Fixes https://bugs.alpinelinux.org/issues/6360

--- a/libmilter/libmilter.h
+++ b/libmilter/libmilter.h
@@ -127,10 +127,10 @@
 # define MI_SOCK_READ(s, b, l)	read(s, b, l)
 # define MI_SOCK_READ_FAIL(x)	((x) < 0)
 # define MI_SOCK_WRITE(s, b, l)	write(s, b, l)
-
-# define thread_create(ptid,wr,arg) pthread_create(ptid, NULL, wr, arg)
 # define sthread_get_id()	pthread_self()

+extern int thread_create(pthread_t *ptid, void *(*wr) (void *), void *arg);
+
 typedef pthread_mutex_t smutex_t;
 # define smutex_init(mp)	(pthread_mutex_init(mp, NULL) == 0)
 # define smutex_destroy(mp)	(pthread_mutex_destroy(mp) == 0)
--- a/libmilter/main.c
+++ b/libmilter/main.c
@@ -16,6 +16,12 @@
 #include <fcntl.h>
 #include <sys/stat.h>

+int thread_create(pthread_t *ptid, void *(*wr) (void *), void *arg) {
+	pthread_attr_t attr;
+	pthread_attr_init(&attr);
+	pthread_attr_setstacksize(&attr,256*1024);
+	return pthread_create(ptid, &attr, wr, arg);
+}

 static smfiDesc_ptr smfi = NULL;