Add 'compile' builtin command for Eshell

Add 'compile' builtin command for Eshell

* lisp/eshell/em-unix.el (eshell-compile, eshell/compile): New
functions.
(eshell/make, eshell-grep): Use 'eshell-compile'.
(eshell/glimpse): It's no longer necessary to let-bind 'null-device';
'eshell-grep' no longer calls 'grep' (the Lisp function), which needed
'null-device' to be nil for this case.

* test/lisp/eshell/em-unix-tests.el: New file.

* doc/misc/eshell.texi (Built-ins): Document the 'compile' builtin.

* etc/NEWS: Announce this change (bug#65273).