MINOR: fd: Use closefrom() as my_closefrom() if supported.

Add a new option, USE_CLOSEFROM. If set, it is assumed the system provides
a closefrom() function, so use it.
It is only implicitely used on FreeBSD for now, it should work on
OpenBSD/NetBSD/DragonflyBSD/Solaris too, but as I have no such system to
test it, I'd rather leave it disabled by default. Users can add USE_CLOSEFROM
explicitely on their make command line to activate it.

Makefile

@@ -374,6 +374,7 @@ ifeq ($(TARGET),freebsd)
   USE_LIBCRYPT   = implicit
   USE_THREAD     = implicit
   USE_CPU_AFFINITY= implicit
+  USE_CLOSEFROM  = implicit
 else
 ifeq ($(TARGET),osx)
   # This is for Mac OS/X
@@ -682,6 +683,10 @@ endif
 endif
 endif
 
+ifneq ($(USE_CLOSEFROM),)
+OPTIONS_CFLAGS += -DUSE_CLOSEFROM
+endif
+
 ifneq ($(USE_LUA),)
 check_lua_lib = $(shell echo "int main(){}" | $(CC) -o /dev/null -x c - $(2) -l$(1) 2>/dev/null && echo $(1))
 check_lua_inc = $(shell if [ -d $(2)$(1) ]; then echo $(2)$(1); fi;)

src/fd.c

@@ -468,7 +468,13 @@ void fd_process_cached_events()
 	fdlist_process_cached_events(&fd_cache);
 }
 
-#if defined(ENABLE_POLL)
+#if defined(USE_CLOSEFROM)
+void my_closefrom(int start)
+{
+	closefrom(start);
+}
+
+#elif defined(ENABLE_POLL)
 /* This is a portable implementation of closefrom(). It closes all open file
  * descriptors starting at <start> and above. It relies on the fact that poll()
  * will return POLLNVAL for each invalid (hence close) file descriptor passed