syscalls: Make the cmd field of fcntl an enum

syscalls: Make the cmd field of fcntl an enum

The fcntl system call has a cmd parameter which is one of multiple
values defined in the linux/fcntl.h file. This field is now an
enumeration of the various possible values.

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

[...] syscall_entry_fcntl: { cpu_id = 3 }, { fd = 35,
cmd = ( "F_GETFL" : container = 3 ), arg = 140388746332169 }

Change-Id: I06f511d5dba986aaf01b50057cbe6eac789bbea7
Signed-off-by: Geneviève Bastien <gbastien+lttng@versatic.net>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>