inetd: final round of trivial cleanup, NFC

Highlights:

• Use MAX() for maxsock raising; small readability improvement IMO
• malloc(3) + memset(3) -> calloc(3) where appropriate
• stop casting the return value of malloc(3)
• mallloc(3) -> reallocarray(3) where appropriate

A future change may enter capability mode when forking for some of the
built-in handlers.