dup2: Make code more maintainable. * lib/dup2.c (dup2_nothrow): New function, extracted from rpl_dup2. (rpl_dup2): Use it. * m4/dup2.m4 (gl_PREREQ_DUP2): New macro. * modules/dup2 (configure.ac): Invoke it. Reported by Paul Eggert.