[libc] Add isprint()

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/core/debug.c

 #include <stdio.h>
 #include <stdint.h>
 #include <stdarg.h>
+#include <ctype.h>
 #include <ipxe/console.h>
 
 /**
 			continue;
 		}
 		byte = bytes[i];
-		if ( ( byte < 0x20 ) || ( byte >= 0x7f ) )
-			byte = '.';
-		dbg_printf ( "%c", byte );
+		dbg_printf ( "%c", ( isprint ( byte ) ? byte : '.' ) );
 	}
 	dbg_printf ( "\n" );
 }

src/include/ctype.h

 #define islower(c)	((c) >= 'a' && (c) <= 'z')
 #define isupper(c)	((c) >= 'A' && (c) <= 'Z')
 #define isxdigit(c)	(isdigit(c) || ((c) >= 'A' && (c) <= 'F') || ((c) >= 'a' && (c) <= 'f'))
+#define isprint(c)	((c) >= ' ' && (c) <= '~' )
 
 static inline unsigned char tolower(unsigned char c)
 {