syntax: adjust the magic strings for the changes since file-5.10

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.

syntax/texinfo.nanorc

@@ -2,7 +2,7 @@
 
 syntax "texinfo" "\.texi$"
 header "^\\input texinfo"
-magic "Texinfo source text"
+magic "Texinfo source"
 comment "@c "
 
 # Command arguments, trailing and enclosed.

syntax/xml.nanorc

@@ -2,7 +2,7 @@
 
 syntax "xml" "\.([jrsx]html?|jnlp|mml|pom|rng|sgml?|svg|w[as]dl|wsdd|xjb|xml|xs(d|lt?)|xul)$"
 header "<\?xml.*version=.*\?>"
-magic "(XML|SGML) (sub)?document text"
+magic "(XML|SGML) (sub)?document"
 comment "<!--|-->"
 
 # The entire content of the tag: