Implicitly support the /dev/fd symlink and friends

Bash has a very convenient feature that is called process substitution
(e.g. `diff -u <(seq 0 10) <(seq 1 11)`). To make this work, Bash
requires the `/dev/fd` symlink to exist, and MSYS2 and Cygwin therefore
create this symlink (together with the `stdin`, `stdout` and `stderr`
ones) upon start-up.

This strategy is a bit incompatible with Git for Windows, which wants to
install the pseudo root into a write-protected area, concretely in
`C:\Program Files\Git`.

This strategy is _also_ incompatible with the idea of providing MinGit
as a `.zip` file (because there is no standard way to represent symlinks
in `.zip` files, and besides, older Windows versions would potentially
lack support for them anyway).

Let's side-step this completely by creating those symlinks implicitly,
similar to the way `/dev/` is populated with special devices.

The diff is misleadlingly large due to the re-generated `devices.cc`.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

winsup/cygwin/Makefile.am

@@ -230,6 +230,7 @@ DLL_FILES= \
 	fhandler_console.cc \
 	fhandler_cygdrive.cc \
 	fhandler_dev.cc \
+	fhandler_dev_fd.cc \
 	fhandler_disk_file.cc \
 	fhandler_dsp.cc \
 	fhandler_fifo.cc \