Created px4_access to handle check of virtual files

uORBManager_posix.cpp did a stat to see if a file exists but the
file is actually a virtual file. Using stat was incorrect because
it required a stat buffer that was never used. The POSIX access
function is a better choice so I created a px4_access version to
handle virtual files.

Signed-off-by: Mark Charlebois <charlebm@gmail.com>

src/platforms/px4_posix.h

@@ -67,6 +67,7 @@ typedef struct pollfd px4_pollfd_struct_t;
 #define px4_read 	_GLOBAL read
 #define px4_poll 	_GLOBAL poll
 #define px4_fsync 	_GLOBAL fsync
+#define px4_access 	_GLOBAL access
 
 #elif defined(__PX4_POSIX)
 
@@ -96,6 +97,7 @@ __EXPORT ssize_t	px4_write(int fd, const void *buffer, size_t buflen);
 __EXPORT int		px4_ioctl(int fd, int cmd, unsigned long arg);
 __EXPORT int		px4_poll(px4_pollfd_struct_t *fds, nfds_t nfds, int timeout);
 __EXPORT int		px4_fsync(int fd);
+__EXPORT int		px4_access(const char *pathname, int mode);
 
 __END_DECLS
 #else