* Improved completion support for files with spaces

Filenames with spaces, both generally and for session names, will now
complete as expected.

* Misc shell script improvements

See discussion on:
1cca9e4b72

This reverts commit 547d87bfc2.

Known issues:
- To optimize dentfill() on most architectures we don't fstatat() with
  AT_SYMLINK_NOFOLLOW to get the inode number of the symlink itself.
- The current hash size serves the purpose of one filesystem. To serve
  more and have unique bits, we have to add more bits to prefix dev ID
  to the inode.  Memory consumption will be significant.

This will be reverted in next commit.

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.