[ash] Add fg command for simple job control to ash

[ghaerr]

Adds fg command to ash for simple job control. Use in conjunction with ^Z (terminal stop) and jobs command to restart stopped jobs into foreground.

ELKS ash doesn't implement full job control. In particular, job control doesn't work well or at all with programs that are stopped while waiting for input, due to problems related to simultaneous reading of stdin by the shell and child programs. This implementation of fg essentially just reads the list of stopped jobs and executes a kill -CONT <pid>. Use jobs to list all stopped or background jobs.

@Mellvik This is the fg command enhancement you requested (the previously described ash enhancements must also be included).

Screenshot showing sample out program (output's X's) being run and stopped twice, then restarted using fg:

[Mellvik]

Nice, @ghaerr, this is good. an extra treat (!) would be to add '%' syntax to the command, (fg %2). Its may not make sense at this point but let me ask anyway: Is fg w/o parameters supported? Not having '+' and '-' available in the jobs listing, would fg alone simply take the highest job #?

[ghaerr]

@Mellvik, all your requested treats have been added in #2574.

fg without parameters will reference the most recently-run or accessed stopped job.

So everything basically now works as expected.