introduce cngets, a method for kernel to read a string from console

This is intended as a replacement for libkern's gets and mostly borrows
its implementation.  It uses cngrab/cnungrab to delimit kernel's access
to console input.

Note: libkern's gets obviously doesn't share any bits of implementation
iwth libc's gets.  They also have different APIs and the former doesn't
have the overflow problems of the latter.

Inspired by:	bde
MFC after:	2 months

sys/kern/kern_cons.c

@@ -407,6 +407,55 @@ cncheckc(void)
 	return (-1);
 }
 
+void
+cngets(char *cp, size_t size, int visible)
+{
+	char *lp, *end;
+	int c;
+
+	cngrab();
+
+	lp = cp;
+	end = cp + size - 1;
+	for (;;) {
+		c = cngetc() & 0177;
+		switch (c) {
+		case '\n':
+		case '\r':
+			cnputc(c);
+			*lp = '\0';
+			cnungrab();
+			return;
+		case '\b':
+		case '\177':
+			if (lp > cp) {
+				if (visible) {
+					cnputc(c);
+					cnputs(" \b");
+				}
+				lp--;
+			}
+			continue;
+		case '\0':
+			continue;
+		default:
+			if (lp < end) {
+				switch (visible) {
+				case GETS_NOECHO:
+					break;
+				case GETS_ECHOPASS:
+					cnputc('*');
+					break;
+				default:
+					cnputc(c);
+					break;
+				}
+				*lp++ = c;
+			}
+		}
+	}
+}
+
 void
 cnputc(int c)
 {

sys/sys/cons.h

@@ -121,6 +121,7 @@ void	cngrab(void);
 void	cnungrab(void);
 int	cncheckc(void);
 int	cngetc(void);
+void	cngets(char *, size_t, int);
 void	cnputc(int);
 void	cnputs(char *);
 int	cnunavailable(void);