Allow serial-term to take an optional argument for line-mode

* lisp/term.el (serial-term): Allow taking an optional argument to
avoid term-raw-mode (bug#24922).

* doc/lispref/processes.texi (Serial Ports): Document it.
1 job for master in 90 minutes and 2 seconds (queued for 5 seconds)
Status Job ID Name Coverage
  Test
failed #2740
test-all

01:30:02

 
Name Stage Failure
failed
test-all Test The script exceeded the maximum execution time set for the job
make -C test check-expensive
make[1]: Entering directory '/builds/emacs/emacs/test'
rm -f ./*.tmp
make[2]: Entering directory '/builds/emacs/emacs/test'
make[3]: Entering directory '/builds/emacs/emacs/test'
ELC lisp/net/tramp-tests.elc
GEN lisp/net/tramp-tests.log
Pulling docker image gitlab/gitlab-runner-helper:x86_64-d0b76032 ...
ERROR: Job failed: execution took longer than 1h30m0s seconds