unix: avoid using non-portable `read -p'
unix: avoid using non-portable `read -p'

`read -p' is not POSIX-compatible, but bash-compatible. See [0], [1].
Thus replace it with `printf' and `read' combo that does the same,
but still conforms with POSIX.

[0]: http://pubs.opengroup.org/onlinepubs/9699919799/utilities/read.html
[1]: https://www.gnu.org/software/bash/manual/bashref.html#index-read

elecalion/vim-gutentags,ludovicchabant/vim-gutentags