substitute, g, vimgrep: check leave empty pattern

Vim is using empty pattern for @/ (the previous pattern).
use very magic patterns: well we should, isn't it ;)
remove a debug message
add VeryMagicSearchArg

For :edit +/pattern type commands:
    - add '\\v' to pattern (g:VeryMagicSearchArg)
    - escape '\' in pattern (g:VeryMagicEscapeBackslashesInSearchArg)