Fix Savannah bug #20273, xargs -E with seekable stdin.

Fix Savannah bug #20273, xargs -E with seekable stdin.
* import-gnulib.config (modules): Sort, add closein.
* xargs/testsuite/Makefile.am (EXTRA_DIST_EXP, EXTRA_DIST_XO)
(EXTRA_DIST_XI): Add new test.
* xargs/testsuite/config/unix.exp (xargs_start): Support optional
argument to allow test to run a subshell.
* xargs/testsuite/inputs/sv-bug-20273.xi: New file.
* xargs/testsuite/xargs.posix/sv-bug-20273.xo: Likewise.
* xargs/testsuite/xargs.posix/sv-but-20273.exp: Likewise.
* xargs/xargs.c (main): Use close_stdin, not close_stdout.
* NEWS: Document the fix.

Added a maintenance manual.
* doc/Makefile.am (info_TEXINFOS): Added doc/find-main.texi.
* doc/find.texi (Introduction): Fixed typo.