Adding "[abc]*" does not restrict the recognized header line in any way.
Also, improve the header-line regex for shell scripts, because it should
not match "barunscript" (for example).
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.
