popen: add cwd run option

Sometimes it is required to run external command in a specific directory. At this moment it is required to do something like this

local cwd = fio.cwd()
fio.chdir(path)
pcall(popen.new, { cmd })
fio.chdir(cwd)

or if you are running with shell flag it could be done with less code

cmd = string.format('cd %s && %s', path, cmd)
popen.new({ cmd }, { shell = true })

Would be nice to have cwd option so it all would look like this

popen.new({ cmd }, { cwd = path })

Start at the popen.new entry point and inspect how its command options are handled, using the existing fio.cwd and fio.chdir examples for context. The work is done when popen.new accepts cwd and runs the external command in that directory without requiring the caller to change the process working directory.