Replace makefs' hand-rolled unescaping with strunvis.

[benno]

Summary

mtree path names and link attributes are encoded, generally using strvis. Newer
versions of mtree will use C-style escapes but previously the accepted form was
octal escapes. makefs' mtree code spots the C-style escapes but fails to deal
with octal escapes correctly.

Remove mtree's escape-decoding code (except for a few instances where it's
needed) and instead pass pathnames and link targets through strunvis prior to
use.

[marcel]

Two comments:

1. I see that there's an unvis(3) function as well. That seems like a good way to handle escape sequences in read_word() and not have callers of read_word() handle escape characters.
2. makefs needs to be portable. Do you know if vis(3) exists on macOS and Linux?

usr.sbin/makefs/mtree.c

570 ↗

(On Diff #32328)

Stray assignment?
This is a dereference of node->symlink before checking for a NULL pointer...

965 ↗

(On Diff #32328)

Maybe just write:

if (strnunvis(pathspec, PATH_MAX, pathtmp) == -1) {
    error = errno;
    goto out;
}

That avoids having to reset error and avoids confusion about the return value of strnunvis: it's not an error...

[benno]

• Remove left-over line from debugging.
• Simplify strnunvis call in read_mtree_spec.

[marcel]

LGTM...