Add setter for Program flags

[Tim---]

The ProgramFlags are set by the program_from_* functions, and cannot be modified. This makes several features of drgn unavailable if used as a library.

This patch adds write access to the program flags.

In practice, I use qemu-system and gdb to debug a vmlinux.
Changing the program flags allows me to use a Program created with the default constructor to use Linux specific functions (eg: getting threads).

import gdb
from drgn import Architecture, Platform, Program, ProgramFlags

def read_mem(address, count, offset, physical):
    return gdb.inferiors()[0].read_memory(address, count)

prog = Program(Platform(Architecture.I386))
prog.flags |= ProgramFlags.IS_LINUX_KERNEL
prog.add_memory_segment(1, (1<<32)-1, read_mem)
prog.load_debug_info(['vmlinux'])

for thread in prog.threads():
    print(thread.object.comm)