Assign 1 to `_POSIX_C_SOURCE` to use `fdopen(3)`
Add option handling

options are ...
* `-h|--help`: print usage
* `-i|--ip`: specify ip version
* `-p|--port`: specify destination port
Rename function

_run    ->
daytime
Pass default_host and default_port to function `daytime` as actual parameter
Handle errors in `connect`

Add error handling function `safe_connect`
Extract method `create_connection`

It returns socket descriptor of connection
Fix default_ipv6_host

2001:200:180:430:216:3eff:fe49:c10e (login1) ->
2001:200:180:430:216:3eff:fe0f:19b  (login2)
Change variable type of `default_port`

int   ->
short
Support ipv6

But not sure that login2.sekiya-lab.info supports ipv6 daytimed
Support option argument `host`

Change usage print message
Support option argument `i` and `p`