elfctl: prefix disable flags with "no"

Some ELF feature flags indicate a request to opt-out of some feature,
for example NT_FREEBSD_FCTL_ASLR_DISABLE indicates that ASLR should be
disabled for the tagged binary. Using "aslr" as the short name for the
flag is confusing as it seems to indicate a request for ASLR to be
enabled. Rename "noaslr", and make a similar change for other opt-out
flags.

Reviewed by: bapt, manu, markj
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D28139

(cherry picked from commit c763f99d11fdc9641308124c4a030c90b6a7fdbb)