Rename handle to fd.

Pretty much everywhere handle is used what is really meant is file descriptor (fd). This terminology got migrated over from Perl and is just not quite correct, or at least not as correct as fd.

There were also plenty of places fd was used so now all uses are consistent.

The Perl code was not updated but might be in a future commit.

test/code-count/file-type.yaml

@@ -739,6 +739,22 @@ src/common/io/bufferWrite.h:
   class: core
   type: c/h
 
+src/common/io/fdRead.c:
+  class: core
+  type: c
+
+src/common/io/fdRead.h:
+  class: core
+  type: c/h
+
+src/common/io/fdWrite.c:
+  class: core
+  type: c
+
+src/common/io/fdWrite.h:
+  class: core
+  type: c/h
+
 src/common/io/filter/buffer.c:
   class: core
   type: c
@@ -783,22 +799,6 @@ src/common/io/filter/size.h:
   class: core
   type: c/h
 
-src/common/io/handleRead.c:
-  class: core
-  type: c
-
-src/common/io/handleRead.h:
-  class: core
-  type: c/h
-
-src/common/io/handleWrite.c:
-  class: core
-  type: c
-
-src/common/io/handleWrite.h:
-  class: core
-  type: c/h
-
 src/common/io/http/client.c:
   class: core
   type: c