implement getusershell, etc. legacy functions

I actually wrote these a month ago but forgot to integrate them. ugly,
probably-harmful-to-use functions, but some legacy apps want them...

include/unistd.h

@@ -163,6 +163,9 @@ char *get_current_dir_name(void);
 int daemon(int, int);
 int getdomainname(char *, size_t);
 int getdtablesize(void);
+void setusershell(void);
+void endusershell(void);
+char *getusershell(void);
 #endif
 
 #define _XOPEN_VERSION          700

src/misc/getusershell.c

+#define _GNU_SOURCE
+#include <stdio.h>
+#include <string.h>
+#include <stdlib.h>
+
+static const char defshells[] = "/bin/sh\n/bin/csh\n";
+
+static char *line;
+size_t linesize;
+static FILE *f;
+
+void endusershell(void)
+{
+	if (f) fclose(f);
+	f = 0;
+}
+
+void setusershell(void)
+{
+	if (!f) f = fopen("/etc/shells", "rb");
+	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;
+}