Pull up following revision(s) (requested by christos in ticket #741):
lib/libc/stdio/flags.c: revision 1.19
lib/libc/stdio/fdopen.c: revision 1.18
sys/kern/vfs_vnops.c: revision 1.196
lib/libc/stdio/freopen.c: revision 1.20
lib/libc/stdio/fopen.c: revision 1.17
external/bsd/nvi/dist/common/recover.c: revision 1.10
external/bsd/nvi/dist/common/recover.c: revision 1.11
lib/libc/sys/open.2: revision 1.58
sys/sys/fcntl.h: revision 1.49
make the checkok test stricter to avoid races, and use O_REGULAR.
Instead of opening the file and using popen(3), pass the file descriptor
to sendmail directory. Idea and code from Todd Miller.
Add O_REGULAR to enforce opening of only regular files
(like we have O_DIRECTORY for directories).
This is better than open(, O_NONBLOCK), fstat()+S_ISREG() because opening
devices can have side effects.

(msaitoh)