ui-plain.c: fix html and links generated by print_dir() and print_dir_entry()

This patch fixes the following issues:
* the base argument usually isn't zero-terminated, so printing base
  without considering baselen will usually generate random garbage
* when the current url represents a directory but doesn't end in a slash,
  relative urls would be incorrect
* using unescaped paths allows XSS

Signed-off-by: Lars Hjemli <hjemli@gmail.com>