pwrite: Fix test whether it works and make it work on HP-UX 11.11. * m4/pwrite.m4 (gl_FUNC_PWRITE): Use AC_LANG_PROGRAM, not AC_LANG_SOURCE. Extend the test program to catch another HP-UX 11.11 bug. * lib/pwrite.c: Undo 2010-12-31 patch. * doc/posix-functions/pwrite.texi: Document another HP-UX 11.11 bug.