Fold tables.c into options.c and stop exporting the one

Fold tables.c into options.c and stop exporting the one
table (dhcp_options) involved. Provide functions
code_to_name(), name_to_code(), code_to_format() and
replace direct access to dhcp_options with them. Eliminate
unneeded 'struct option'.

Unhook tables.c from Makefile.