popen: add cwd run option

[koshoi]

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 })