maint: use more *-safer gnulib modules

See section "Handling closed standard file descriptors" in the gnulib
manual.

* bootstrip.conf (gnulib_modules): Add fcntl-safer, openat-safer
and unistd-safer.
* find/exec.c: Include "fcntl--.h" to use the *-safer variant of open()
or openat().
* find/pred.c: Likewise.
* locate/locate.c: Likewise.
* xargs/xargs.c: Likewise, and include "unustd--.h" to use the *-safer
variant of pipe().

bootstrap.conf

@@ -93,6 +93,7 @@ gnulib_modules="
     faccessat
     fchdir
     fcntl
+    fcntl-safer
     fdopendir
     fflush
     fileblocks
@@ -131,6 +132,7 @@ gnulib_modules="
     mountlist
     nstrftime
     open
+    openat-safer
     parse-datetime
     pathmax
     perror
@@ -167,6 +169,7 @@ gnulib_modules="
     sys_wait
     timespec
     uname
+    unistd-safer
     unlinkat
     verify
     version-etc

find/exec.c

@@ -30,6 +30,7 @@
 #include "cloexec.h"
 #include "dirname.h"
 #include "error.h"
+#include "fcntl--.h"
 #include "save-cwd.h"
 #include "xalloc.h"
 

find/pred.c

@@ -38,6 +38,7 @@
 #include "areadlink.h"
 #include "dirname.h"
 #include "error.h"
+#include "fcntl--.h"
 #include "fnmatch.h"
 #include "stat-size.h"
 #include "stat-time.h"

locate/locate.c

@@ -84,6 +84,7 @@
 #include "progname.h"
 #include "xalloc.h"
 #include "error.h"
+#include "fcntl--.h"
 #include "human.h"
 #include "dirname.h"
 #include "closeout.h"

xargs/xargs.c

@@ -54,9 +54,11 @@
 /* gnulib headers. */
 #include "closein.h"
 #include "error.h"
+#include "fcntl--.h"
 #include "progname.h"
 #include "quotearg.h"
 #include "safe-read.h"
+#include "unistd--.h"
 #include "xalloc.h"
 
 /* find headers. */