input: discard a verbatim 0x0A or 0x00 byte, depending on the mode

This disallows entering a verbatim ^J (0x0A) when typing text or
search terms, and disallows a verbatim ^@ (0x00) when typing a
filename.  Nano beeps when the disallowed code is attempted.

This addresses https://savannah.gnu.org/bugs/?49897.
4 files changed