fix issues from public functions defined without declaration visible

policy is that all public functions which have a public declaration
should be defined in a context where that public declaration is
visible, to avoid preventable type mismatches.

an audit performed using GCC's -Wmissing-declarations turned up the
violations corrected here. in some cases the public header had not
been included; in others, a feature test macro needed to make the
declaration visible had been omitted.

in the case of gethostent and getnetent, the omission seems to have
been intentional, as a hack to admit a single stub definition for both
functions. this kind of hack is no longer acceptable; it's UB and
would not fly with LTO or advanced toolchains. the hack is undone to
make exposure of the declarations possible.

src/network/ent.c

@@ -1,10 +1,16 @@
+#include <netdb.h>
 #include "libc.h"
 
 void sethostent(int x)
 {
 }
 
-void *gethostent()
+struct hostent *gethostent()
+{
+	return 0;
+}
+
+struct netent *getnetent()
 {
 	return 0;
 }
@@ -14,5 +20,4 @@ void endhostent(void)
 }
 
 weak_alias(sethostent, setnetent);
-weak_alias(gethostent, getnetent);
 weak_alias(endhostent, endnetent);