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

fusl/src/legacy/getusershell.c

+#define _GNU_SOURCE
+#include <stdio.h>
+#include <unistd.h>
+
+static const char defshells[] = "/bin/sh\n/bin/csh\n";
+
+static char *line;
+static size_t linesize;
+static FILE *f;
+
+void endusershell(void)
+{
+        if (f) fclose(f);
+        f = 0;
+}
+
+void setusershell(void)
+{
+        if (!f) f = fopen("/etc/shells", "rbe");
+        if (!f) f = fmemopen((void *)defshells, sizeof defshells - 1, "rb");
+}
+
+char *getusershell(void)
+{
+        ssize_t l;
+        if (!f) setusershell();
+        if (!f) return 0;
+        l = getline(&line, &linesize, f);
+        if (l <= 0) return 0;
+        if (line[l-1]=='\n') line[l-1]=0;
+        return line;
+}