Add unit tests for k5_parse_host_string

Add unit tests for k5_parse_host_string

Make is_string_numeric() visible outside of parse_host_string.c as
k5_is_string_numeric() so it can be tested.  Make
k5_parse_host_string() return an error when address begins with ':',
for consistency with APR's apr_parse_addr_port().

[ghudson@mit.edu: squashed three commits; added t_parse_host_string to
.gitignore and clean rule; clarified commit message]