Keep subcommands in pipelines from clobbering the head/tail processes

Keep subcommands in pipelines from clobbering the head/tail processes

* lisp/eshell/esh-cmd.el (eshell-execute-pipeline): Use 'make-symbol'
for headproc and tailproc.
(eshell-do-pipelines, eshell-do-pipelines-synchronously): Adapt to the
above.

* test/lisp/eshell/eshell-tests.el (eshell-test/pipe-subcommand)
(eshell-test/pipe-subcommand-with-pipe): New test.

* doc/misc/eshell.texi (Bugs and ideas): Remove item about piping to
process from loop; this commit fixes it (bug#55590).