Replace license boilerplate with single line SPDX comments. This gets rid of existing copyright templates in favor of just the one-liner SPDX (Software Package Data Exchange) notice. All files are licensed under the GPL-2.0, so the same tag is added to each file. No copyright is changed by this commit. Several files (mostly the very short ones) did not contain a license text so far. By default all files without license information are under the default license of this package, which is GPL version 2. This commit adds the missing SPDX line so that now all files except dss.css, index.html.in, INSTALL, NEWS and README have it. We also remove author and copyright year, since the author is the same everywhere, and the year hasn't been updated any more since at least six years. Accurate information is available from the git log. The COPYING file can also be removed because the license text at https://spdx.org/licenses/GPL-2.0.html is immutable.
run: Improve error diagnostics for chdir(2) failure. In run mode, if the destination directory does not exist, dss prints "No such file or directory" and exits, without telling the user (a) it was a failed chdir(2) call that caused the error, and (b) the name of the directory. This patch adds an error message containing this information. Since there is only one caller of dss_chdir(), let's get rid of this public function in file.c and call chdir() directly from change_to_dest_dir() of dss.c.
Assorted typo fixes in comments.
Change email address and URLs. The web and email service of systemlinux.org was down for two weeks. Meanwhile the dss web pages found a new home at the MPI campus of Tübingen. This commit changes the email addresses and the home page URL.
Merge branch 't/rename_system_files' Has been cooking for several weeks. The conflicts daemon.c exec.c file.c ipc.c sig.c are trivial and can be resolved easily.
Rename source files which also exist as system headers. As pointed out by Daniel Richard G. some of the dss header files are named the same as system header files. This patch renames these headers as well as their corresponding .c files. Specifically, error.h, fd.h, signal.h, string.h and time.h become err.h, file.h, sig.h, str.h and tv.h.