Add a "fork" of musl as //fusl.

fusl/src/misc/initgroups.c

Index: fusl/src/misc/initgroups.c
diff --git a/fusl/src/misc/initgroups.c b/fusl/src/misc/initgroups.c
new file mode 100644
index 0000000000000000000000000000000000000000..922a958142b0ad7b1f331f30725ecec29b91a309
--- /dev/null
+++ b/fusl/src/misc/initgroups.c
@@ -0,0 +1,11 @@
+#define _GNU_SOURCE
+#include <grp.h>
+#include <limits.h>
+
+int initgroups(const char *user, gid_t gid)
+{
+	gid_t groups[NGROUPS_MAX];
+	int count = NGROUPS_MAX;
+	if (getgrouplist(user, gid, groups, &count) < 0) return -1;
+	return setgroups(count, groups);
+}