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/linux/brk.c

@@ -1,3 +1,5 @@
+#define _BSD_SOURCE
+#include <unistd.h>
 #include <errno.h>
 #include "syscall.h"
 

src/linux/clone.c

@@ -1,5 +1,7 @@
+#define _GNU_SOURCE
 #include <stdarg.h>
 #include <unistd.h>
+#include <sched.h>
 #include "pthread_impl.h"
 #include "syscall.h"
 

src/linux/sbrk.c

@@ -1,3 +1,5 @@
+#define _BSD_SOURCE
+#include <unistd.h>
 #include <stdint.h>
 #include <errno.h>
 #include "syscall.h"