sudo(8) (and env, nice, and other entries in \$precommand_options)
resolve their target command via execvp(3)-style lookup, which never
searches \$path for a name containing a slash -- unlike the shell's
own PATH_DIRS option. So "sudo foo/bar" was highlighted as a valid
command whenever the user had PATH_DIRS set and some \$path element
made "foo/bar" resolvable that way, even though sudo itself would
fail to find it.
:sudo_opt: already marks every word from a recognised precommand up
to and including its actual command word (for any precommand in
\$precommand_options, not just sudo). Key off that existing marker to
shadow PATH_DIRS out of \$options_to_set for just those two
_zsh_highlight_main__type calls, via an anonymous-function scope so
the shadow reverts automatically and normal command-word classification
elsewhere in the line is unaffected.
Fixes#595.
Manish Tiwari