Support setting umask symbolically in Eshell

Support setting umask symbolically in Eshell

* lisp/eshell/em-basic.el (eshell/umask): Handle setting umask
symbolically, and make setting umask take precedence over "-S".

* test/lisp/eshell/em-basic-tests.el
(em-basic-test/umask-print-numeric, em-basic-test/umask-read-symbolic,
em-basic-test/umask-set): Rename to...
(em-basic-test/umask/print-numeric)
(em-basic-test/umask/print-symbolic, em-basic-test/umask/set-numeric):
... these.
(em-basic-test/umask/set-symbolic, em-basic-test/umask/set-with-S):
New tests.

* etc/NEWS: Announce this change.