Also removed another stray cat

1. getplugs should work even if nnn is not run
2. Remove redundant ./ prefix
3. Update plugin docs

References:
1. http://man7.org/linux/man-pages/man2/posix_fadvise.2.html
2. https://www.oreilly.com/library/view/linux-system-programming/9781449341527/ch04.html

Remove extra static string.

* getpulgs now asks before overwriting

* Forgot a space

This reverts commit 407b690404.

- press pick plugin key
- navigate away from the plugin directory (to ~, say)
- try to open any file, fails

When we are in run plugin mode, but detect we are in a different
context or a non-plugin directory, just cancel the runplgin mode
and continue as usual.

It also cleans up the plugin run function.

xargs couldn't call function `dnd()` and I couldn't find a portable way
to do it, so I replaced it with a variable which I guess should work
fine.