Improve logging of reserved labels.

Improve logging of reserved labels.

Print "exp-null" and "imp-null" instead of "0" and "3", for example. Also,
remove print_label() and print_pw_type() from ldpctl.c and use the
equivalent functions from ldpd's log.c.

While here, be more paranoid and use UINT32_MAX instead of UINT_MAX
for NO_LABEL.