Also, trim some whitespace and group one rule better.
Also, fix a stray closing parenthesis in the JSON syntax,
and add the missing slash to the possible escaped characters.
(Reference: https://www.json.org/json-en.html.)
Also, improve the ending of multiline strings in the Rust syntax.
Since file-5.10 (end of 2011), libmagic identifies a C file in most
cases as "C source" instead of as "C program". Nano's magic strings
for some other files didn't match any more what file-5.32 currently
produces, either. So, they have been adjusted, new ones added, and
old ones deleted.
This fixes https://savannah.gnu.org/bugs/?52445.
Benno Schulenberg