Revisit afprun functionality (preexec, postexec)

[rdmark]

Like we did with 2.x in #526 I think afprun functionality should be removed from 3.x as well. Including the preexec / postexec, as well as the nostat options.

As with previous feature deprecations in 3.x, let me ask the community on the mailing lists about their use cases.

[rdmark]

Inconclusive feedback from the community so far. Thread starts at https://sourceforge.net/p/netatalk/mailman/message/47401127/

@dgsga What do you think? There is an argument by Ralph that there is a general usecase for this feature, but noone in the community spoke up to say that they actually used it.

[rdmark]

For this ticket I want to pivot to:

1. Understanding what the concrete use cases are for this functionality. What kinds of volume pre-mount and post-dismount steps are actually executed? Is root privileges actually required?
2. Revisit how this functionality is implemented (in particular around root access.)

[ghost]

My take on it is that afprun functionality should be removed for this one reason alone:
"This code constitutes a major opportunity to run arbitrary shell commands (with root privileges) on the host, with all sorts of security implications."

[rdmark]

How about the take the middle road of removing root preexec and root postexec while keeping the non-root functionality?