inet: build all res_* and ns_* functions conditionally

inet: build all res_* and ns_* functions conditionally

This patch changes the build system so that all DNS resolver related
functions (ie, those starting with dn_, ns_ and res_) are only built if
UCLIBC_HAS_RESOLVER_SUPPORT is set. This means that the libc will lack
support for things that existed in the library before unless this new
config option is set. On the other hand, if users really need support
for resolver functions, they now get a more complete set, and the
library is smaller than before it is deselected.

Signed-off-by: Daniel Mack <zonque@gmail.com>