notmuch-emacs-mua: non-forking escape () usage with backslash '\' escape

notmuch-emacs-mua: non-forking escape () usage with backslash '\' escape

Use the printf -v convention to give output variable as argument
to escape () function so no subshell needs to be executed for
escaping input. The '-v' option to escape () is just syntactic
sugar for better understanding.

Also, backslash is now escaped with another backslash for emacs. This
ie especially important at the end of string.

`echo` is no longer used to write escaped output -- it might interpret
the escapes itself.