New shell-mode command to narrow to the command under point

* lisp/shell.el (shell--prompt-end-position)
(shell--prompt-begin-position): Helper functions.

* lisp/shell.el (shell-narrow-to-prompt): New command and
keystroke (bug#41784).
1 job for master in 25 minutes and 28 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
failed #8317
test-all

00:25:28

 
Name Stage Failure
failed
test-all Test
  ELC      erc/erc-netsplit.elc
ELC erc/erc-networks.elc
ELC erc/erc-notify.elc
ELC erc/erc-page.elc
ELC erc/erc-pcomplete.elc
ELC erc/erc-replace.elc
ELC erc/erc-ring.elc
ELC erc/erc-services.elc
ERROR: Job failed: exit code 137