syscalls: Make the flags and mode fields of open[at] enumerations

syscalls: Make the flags and mode fields of open[at] enumerations

The open and openat system call have a flags and mode fields, whose values
are defined in the linux/fcntl.h file. These fields are now
enumerations that can be read as a bit field enum, to make the values more
readable / meaningful.

Here's an example babeltrace output of the open system call:

[...] syscall_entry_openat: { cpu_id = 0 }, { dfd = -100,
filename = "/tmp/edg0_383407",
flags = ( "O_RDWR" | "O_CREAT" | "O_TRUNC" : container = 578 ),
mode = ( "S_IWOTH" | "S_IROTH" | "S_IWGRP" | "S_IRGRP" |
   "S_IWUSR" | "S_IRUSR" : container = 438 ) }

Change-Id: Id7a516670b03e52fc75f9ff3c6ba8114c61a3865
Signed-off-by: Geneviève Bastien <gbastien@versatic.net>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>