feat(osutil.Mkdir): add the chmod flag

[IronCore864]

Add a new Chmod flag to MkdirOptions so that an explicit chmod command is executed on all directories created by osutil.Mkdir.

Closes https://github.com/canonical/pebble/issues/372.

Manual Tests

Comparison

Tests	Before	After
pebble mkdir ~/normal -m 777	umask set to 0022, result is 0755, not the same as what the user specifies because of umask	umask set to 0022, result is 0777, not affected by umask
pebble mkdir -p ~/nested/folder -m 777	umask set to 0022, result is 0755 for both parents and children, not the same as what the user specifies because of umask	umask set to 0022, result is 0777 for both parents and children, not affected by umask

Before

Permission is affected by umask settings:

# umask 0022
$ pebble mkdir ~/normal -m 777
$ ll normal/
total 8
drwxr-xr-x  2 ubuntu ubuntu 4096 Jun  6 02:30 ./
$ pebble mkdir -p ~/nested/folder -m 777
$ ll nested/
total 12
drwxr-xr-x  3 ubuntu ubuntu 4096 Jun  6 02:34 ./
drwxr-x--- 19 ubuntu ubuntu 4096 Jun  6 02:34 ../
drwxr-xr-x  2 ubuntu ubuntu 4096 Jun  6 02:34 folder/

After

Permission isn't affected by umask anymore:

# umask 0022
$ pebble mkdir ~/normal -m 777
$ ll normal/
total 8
drwxrwxrwx  2 ubuntu ubuntu 4096 Jun  6 02:31 ./
$ pebble mkdir -p ~/nested/folder -m 777
$ ll nested/
total 12
drwxrwxrwx  3 ubuntu ubuntu 4096 Jun  6 02:32 ./
drwxr-x--- 19 ubuntu ubuntu 4096 Jun  6 02:32 ../
drwxrwxrwx  2 ubuntu ubuntu 4096 Jun  6 02:32 folder/

[benhoyt]

I've tested this manually locally too, and it seems to work nicely.