fcntl-h: default O_SEARCH, O_EXEC to O_PATH if available

fcntl-h: default O_SEARCH, O_EXEC to O_PATH if available

Linux kernel 2.6.39 introduced O_PATH (see
<http://lwn.net/Articles/433854/>) and this is a better fallback
for O_SEARCH and O_EXEC than O_RDONLY, if O_PATH is available.
* doc/posix-headers/fcntl.texi (fcntl.h): Document this.
* lib/fcntl.in.h (O_EXEC, O_SEARCH) [O_PATH]: Default to O_PATH.
* lib/fcntl.in.h (O_ACCMODE):
* tests/test-fcntl-h.c (main):
Do not reject O_ACCMODE merely because it has more than the
minimal number of bits, as POSIX allows extensions here.