realpath: a new program to print the resolved path

realpath: a new program to print the resolved path

This program is compatible with other realpath(1)
implementations, and also incorporates relpath like support,
through the --relative options.  The relpath support
was suggested by Peng Yu, who also provided an initial
implemenation of that functionality.

* AUTHORS: Add my name.
* NEWS: Mention the new command.
* README: Likewise.
* doc/coreutils.texi (realpath invocation): Add realpath info.
* man/Makefile.am (realpath.1): Add dependency.
* man/realpath.x: New template.
* man/.gitignore: Ignore generated man page.
* po/POTFILES.in: Add src/realpath.c.
* src/.gitignore: Exclude realpath.
* src/Makefile.am (EXTRA_PROGRAMS): Add realpath.
* src/realpath.c: New file.
* scripts/git-hooks/commit-msg: Add realpath to the list of prefixes.
* tests/Makefile.am (TESTS): Add misc/realpath.
* tests/misc/realpath: New file.