Validate ssh files for auto completion

* Validate that ~/.ssh/{config,known_hosts} exist before adding using
  them for auto completion
Validate power down  aliases before assigning

* Validate that systemctl exists before assigning alias to
  reboot and shutdown
PS1 data and format customizations

* nested sessions count
* background process count
* ssh marker
* Prompt root color dynamically
* Prompt user character set dynamically
Replace ctags generation map <leader>0

* Switched '0' for 'F10' since  <leader><F10> would not be recognized by vim
Starting with relative line numbers by default