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

fusl/src/legacy/getusershell.c

Index: fusl/src/legacy/getusershell.c
diff --git a/fusl/src/legacy/getusershell.c b/fusl/src/legacy/getusershell.c
new file mode 100644
index 0000000000000000000000000000000000000000..5fecdec2e44196f19a53dc4fd15018abc47bbb8c
--- /dev/null
+++ b/fusl/src/legacy/getusershell.c
@@ -0,0 +1,32 @@
+#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;
+}