implement disown builtin

Closes #2810.

The syntax mirrors that of zsh.

doc_src/disown.txt

@@ -0,0 +1,26 @@
+\section disown disown - remove a process from the list of jobs
+
+\subsection disown-synopsis Synopsis
+\fish{synopsis}
+disown [ PID ... ]
+\endfish
+
+\subsection disown-description Description
+
+`disown` removes the specified <a href="index.html#syntax-job-control">job</a> from the list of jobs. The job itself continues to exist, but fish does not keep track of it any longer.
+
+Jobs in the list of jobs are sent a hang-up signal when fish terminates, which usually causes the job to terminate; `disown` allows these processes to continue regardless.
+
+If no process is specified, the most recently-used job is removed (like `bg` and `fg`).  If one or more `PID`s are specified, jobs with the specified process IDs are removed from the job list. Invalid jobs are ignored and a warning is printed.
+
+If a job is stopped, it is sent a signal to continue running, and a warning is printed. It is not possible to use the `bg` builtin to continue a job once it has been disowned.
+
+The PID of the desired process is usually found by using <a href="index.html#expand-process">process expansion</a>, which can specify jobs or search by process name.
+
+`disown` returns 0 if all specified jobs were disowned successfully, and 1 if any problems were encountered.
+
+\subsection disown-example Example
+
+`firefox &; disown` will start the Firefox web browser in the background and remove it from the job list, meaning it will not be closed when the fish process is closed.
+
+`disown (jobs -p)` removes all jobs from the job list without terminating them.