Add Zstandard compression support for etags

* lib-src/etags.c: (compressors): Add zstd support.
(print_language_names): Report zstd support.
* doc/man/etags.1: Update doc.

doc/man/etags.1

@@ -64,7 +64,7 @@ Files specified with absolute file names will be recorded
 with absolute file names.  Files generated from a source file\-\-like
 a C file generated from a source Cweb file\-\-will be recorded with
 the name of the source file.
-Compressed files are supported using gzip, bzip2, and xz.
+Compressed files are supported using gzip, bzip2, xz, and zstd.
 The programs recognize the language used in an input file based on its
 file name and contents.  The \fB\-\-language\fP switch can be used to force
 parsing of the file names following the switch according to the given

etc/NEWS

@@ -584,6 +584,9 @@ The mode is automatically enabled in files that start with the
 use the new 'fileloop-initialize' and 'fileloop-continue' functions
 instead.
 
++++
+*** etags is now able to read Zstandard-compressed files.
+
 ** bibtex
 
 ---

lib-src/etags.c

@@ -519,6 +519,7 @@ static compressor compressors[] =
   { "GZ", "gzip -d -c"},
   { "bz2", "bzip2 -d -c" },
   { "xz", "xz -d -c" },
+  { "zst", "zstd -d -c -q" },
   { NULL }
 };
 
@@ -861,7 +862,7 @@ followed by the name of an interpreter.  If no such sequence is found,\n\
 Fortran is tried first; if no tags are found, C is tried next.\n\
 When parsing any C file, a \"class\" or \"template\" keyword\n\
 switches to C++.");
-  puts ("Compressed files are supported using gzip, bzip2, and xz.\n\
+  puts ("Compressed files are supported using gzip, bzip2, xz, and zstd.\n\
 \n\
 For detailed help on a given language use, for example,\n\
 etags --help --lang=ada.");