fs: add support for relative ACL changes

fs: add support for relative ACL changes

This change permits "fs setacl" to change ACLs in a relative
manner, rather than just setting rights absolutely as it is
done now.

If a single plus (+) or minus (-) character is appended to
the rights' letters argument, the new rights are computed
relatively to the existing ones.

A few examples should make clear that behaviour:

old rights: rights set: new rights:
-----------------------------------------------
rl a+ rla
rlid idwa- rl
rla write- a
rl write- [none] (ie. entry deleted)
[any] read= rl

As shown in the last example, a '=' character got implemented
also (and for free) as an alternative writing of the current
and default behaviour of just setting an ACL.

FIXES 123962

Change-Id: If15a4ab3c69ec44a42c8746a0b93f5e8b785d61e
Reviewed-on: http://gerrit.openafs.org/4496
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>