include cleanups: remove unused headers and add feature test macros

fix undefined strcpy call in inet_ntop

source and dest arguments for strcpy cannot overlap, so memmove must
be used here. the length is already known from the above loop.

make inet_ntop format v4-mapped ipv6 addresses properly

based on a patch by orc. POSIX actually fails to specify the format of
the ntop conversion; presumably, any output that will correctly
round-trip back via the (well-specified) pton operation is acceptable.
the new behavior is much more convenient than the old, however.

this patch also affects getnameinfo, which is implemented in terms of
inet_ntop and which is the preferred interface for performing this
conversion.

I've also removed some inexplicable cruft (filling the buffer with 'x'
before doing anything) whose origin I was unable to track down.

use restrict everywhere it's required by c99 and/or posix 2008

to deal with the fact that the public headers may be used with pre-c99
compilers, __restrict is used in place of restrict, and defined
appropriately for any supported compiler. we also avoid the form
[restrict] since older versions of gcc rejected it due to a bug in the
original c99 standard, and instead use the form *restrict.

remove some stray trailing space characters

fix ipv6 address printing: 2001 appeared as 201, etc.

initial check-in, version 0.5.0