Refactor host validation and parsing, move methods out of tr_util.[ch]

Refactor host validation and parsing, move methods out of tr_util.[ch]

  * Limit hostname validation to avoiding ambiguity about whether a port
    is part of the string
  * Refactor hostname/port parsing
    - new function is tr_parse_host() in tr_inet_util.c
    - handles both hostname and port
    - works with strings, not TR_NAME
  * Move hostname related methods out of tr_util.c

Changes to make the rest of the codebase work with these updates will be
in the next commit.