Fix aesmd not starting if /dev is mounted as noexec

[assafmo]

See https://lore.kernel.org/linux-sgx/4db41057-910c-b686-0428-474debe382c1@fortanix.com/

[assafmo]

So actually this didn't work for me. Instead of using ExecStartPre I had to create a startup script especially for remounting /dev as exec.

See scrtlabs/SecretNetwork@866949a?diff=split#diff-fc708eba6ce617e52f528df7404127e6R77-R90

I don't know what you'd want to do with this PR or even if you want to support ubuntu != {16.04,18.04}.

Anyway I think the driver should make sure that /dev/sgx and /dev/isgx are mounted as exec.

[andyzyb]

I somewhat agree that it is better to leave this to driver. In some cases there is no AESM installed on the system, then user application needs to handle this?

[assafmo]

I don't think the user application needs to handle this, but the driver. I wouldn't want my apps to remount /dev on my system.
Though if not AESM then I think the driver should handle this.

[haitaohuang]

This is discussed in LKML for upstreaming driver. It needs be a solution from driver

[kaimast]

Thanks for posting the fix @assafmo!