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.
A tag begins not merely with a "<" but it must be followed by an
ASCII alphabetic character or one of "/", "!" and "?".
Further, color all the valid attribute names in red.
Benno Schulenberg