symlink: new module, for Solaris 9 bug symlink("a","link/") mistakenly succeeds. * modules/symlink: New file. * m4/symlink.m4 (gl_FUNC_SYMLINK): Likewise. * lib/symlink.c: Likewise. * m4/unistd_h.m4 (gl_UNISTD_H_DEFAULTS): Add defaults. * modules/unistd (Makefile.am): Substitute them. * lib/unistd.in.h (symlink): Declare replacement. * MODULES.html.sh (File system functions): Mention it. * doc/posix-functions/symlink.texi (symlink): Likewise. * modules/symlink-tests: New test. * tests/test-symlink.c: Likewise. Signed-off-by: Eric Blake <ebb9@byu.net>