Distribute rmt.8; update build system

* NEWS: Update.
* bootstrap (gnulib_extra_files): Remove "missing"
* configure.ac: Do not distribute shar archive.
Define CPIO_MT_COND
* doc/Makefile.am: Always install manpages (mt.1 and rmt.8 --
depending on whether the corresponding programs are built).
* doc/cpio.1: Update.
* doc/mt.1: Update.
* lib/Makefile.am: Use AM_CPPFLAGS instead of INCLUDES.
* src/Makefile.am: Likewise.
* tests/Makefile.am: Likewise.
Improve documentation.

* .gitignore: Update.
* NEWS: Update.
* doc/cpio.1: Rewrite.
* doc/cpio.texi: Major revamp.
* src/main.c (options): Fix sectioning of the help output.
(parse_opt):
* src/util.c: Use PAXEXIT_FAILURE to indicate an error.
Use exit codes consistenly.
Treat UID/GID as numeric if prefixed by + (-R option)

The IDs supplied with the -R option are treated as numeric (without
looking them up in the system database), when prefixed with +.  This
allows to force using numeric value if a user (group) with a numeric
name exists in the database.

Reported by Joshua Briefman <sirgatez@gmail.com>.

* src/userspec.c (parse_user_spec): Use + as an indicator of
a numeric UID/GID.
* doc/cpio.1: Document changes.
* doc/cpio.texi: Likewise.