Replace 48 lines of code with a single inet_pton() call. The previous
handrolled version could not even make use of sscanf(), since that would
not work with a certain antiquated compiler.

It is worth noting that there is a tiny change in behaviour - previously
calling BIO_get_host_ip() with something that looked like it might be a
valid IP address (for example, "1." or even ".") would result in it
returning failure rather than trying a BIO_gethostbyname() - now we'll
always try a BIO_gethostbyname() if it was not a valid IPv4 address.

ok beck@ miod@ deraadt@