This patch cleans up the automatically-generated (?) code and fixes one
case that will always fail due to syntax error.

HAVE_GETHOSTBYADDR_R_5_REENTRANT always failed because of a trailing
character ("int length;q"). Several parameter type and unused variable
warnings popped up. This causes a detection failure with -Werror.

Observe that the REENTRANT cases are exactly the same as their
non-REENTRANT cases except for a `_REENTRANT` macro definition.
Merge all these pieces and build one big main function with different
cases, but reusing variables where logical.

For the cases where the parameters where NULL, I looked at
lib/hostip4.c to get an idea of the parameters types.

void-cast variables such as 'rc' to avoid -Wuninitialized errors.

Signed-off-by: Peter Wu <peter@lekensteyn.nl>