Unify some node handling functions that use TOKEN_NONE.

Unify some node handling functions that use TOKEN_NONE.
* mdoc_word_alloc(), man_word_alloc() -> roff_word_alloc()
* mdoc_word_append(), man_word_append() -> roff_word_append()
* mdoc_addspan(), man_addspan() -> roff_addtbl()
* mdoc_addeqn(), man_addeqn() -> roff_addeqn()
Minus 50 lines of code, no functional change.