[MannXo]

clap strips a = that starts a value attached to a short option, so env -C=/etc pwd ran in /etc instead of ./=/etc. The same went for -a=NAME, clustered forms like -vC=/etc, and -S'-C=/etc pwd'. -C= failed with "cannot change directory to ''", and -vu=FOO unset FOO and exited 0 where GNU rejects it.

The arguments are now passed through preserve_attached_equals before clap, the same approach as preserve_prefix_equals in fmt. It doubles the = after -a, -C or -u so that clap's stripping leaves the value intact. It stops at the first operand or -- and skips detached option values, so arguments meant for the command are untouched. A -u value starting with = now reaches the existing invalid-name check, so the cluster form fails with 125 like -u=FOO already did.

$ mkdir -p /tmp/w/=/etc && cd /tmp/w && env -C=/etc pwd
/private/tmp/w/=/etc
$ env -a=name sh -c 'echo $0'
=name
$ FOO=1 env -vu=FOO true; echo $?
env: cannot unset '=FOO': Invalid argument
125

test_short_option_attached_value_keeps_leading_equals and the new -vu= case in disallow_equals_sign_on_short_unset_option fail on main.

Closes #15754