Handle IPv4 mapped addresses; Detect IPv6 addresses at Installation

[vipsoft]

core/Common.php getIp() uses ip2long(). This only works for ipv4 addresses.

ipv6 addresses result in 0 (and are converted back as "0.0.0.0").

[vipsoft]

Unit tests should ensure these formats are handled:

• 2001:0db8:85a3:0000:0000:8a2e:0370:7334
• 2001:db8:85a3:0:0:8a2e:370:7334 (leading zeros omitted)
• 2001:db8:85a3::8a2e:370:7334 (groups of zeros omitted)
• ::ffff:c000:280 and ::ffff:192.0.2.128(double quad notation for ipv4 mapped addresses)

[vipsoft]

MySQL bigint is 8 bytes and won't accommodate a 128 bit ipv6 address. So... we'll defer comment:2 for post-1.0.

The current proposal is:

• Installation: detect ipv6 REMOTE_ADDR and warn
• core/Common.php: getIp() - accept IPv4-mapped addresses prefixed by ::ffff:

[vipsoft]

(In [1790]) fixes #1110 - warn if IPv6 address detected during Installation; handle IPv4 mapped addresses