mtree: Sync with upstream

[jlduran_gmail.com]

Summary

This updates our differences with upstream.
There is a small caveat regarding the setting of errno = 0 that we believe should not be needed, however out of an abundance of caution, we decided to keep it until further investigation/testing is performed.

---

The commit-by-commit history is on https://github.com/freebsd/freebsd-src/pull/918. It has been "double posted" here for visibility and to reach the right person/people for this review.

Test Plan

# kyua test usr.sbin/nmtree
usr.sbin/nmtree/nmtree_test:mtree_check  ->  passed  [0.029s]
usr.sbin/nmtree/nmtree_test:mtree_convert_C  ->  passed  [0.017s]
usr.sbin/nmtree/nmtree_test:mtree_convert_C_S  ->  passed  [0.016s]
usr.sbin/nmtree/nmtree_test:mtree_convert_D  ->  passed  [0.016s]
usr.sbin/nmtree/nmtree_test:mtree_convert_D_S  ->  passed  [0.021s]
usr.sbin/nmtree/nmtree_test:mtree_create  ->  passed  [0.027s]
usr.sbin/nmtree/nmtree_test:mtree_ignore  ->  passed  [0.031s]
usr.sbin/nmtree/nmtree_test:mtree_merge  ->  passed  [0.019s]
usr.sbin/nmtree/nmtree_test:mtree_nonemptydir  ->  passed  [0.031s]
usr.sbin/nmtree/nmtree_test:mtree_onlyfile  ->  passed  [0.018s]
usr.sbin/nmtree/nmtree_test:mtree_specspec_type  ->  passed  [0.033s]
usr.sbin/nmtree/nmtree_test:netbsd6_check  ->  passed  [0.024s]
usr.sbin/nmtree/nmtree_test:netbsd6_convert_C  ->  passed  [0.020s]
usr.sbin/nmtree/nmtree_test:netbsd6_convert_C_S  ->  passed  [0.019s]
usr.sbin/nmtree/nmtree_test:netbsd6_convert_D  ->  passed  [0.016s]
usr.sbin/nmtree/nmtree_test:netbsd6_convert_D_S  ->  passed  [0.016s]
usr.sbin/nmtree/nmtree_test:netbsd6_create  ->  passed  [0.030s]
usr.sbin/nmtree/nmtree_test:netbsd6_ignore  ->  passed  [0.032s]
usr.sbin/nmtree/nmtree_test:netbsd6_merge  ->  passed  [0.019s]
usr.sbin/nmtree/nmtree_test:netbsd6_nonemptydir  ->  passed  [0.028s]
usr.sbin/nmtree/nmtree_test:netbsd6_onlyfile  ->  passed  [0.018s]

...

21/21 passed (0 failed)

[jlduran_gmail.com]

Note to self:
It will likely land as two separate commits:

1. Vendor import of vendor/NetBSD/mtree.
2. Vendor import of vendor/NetBSD/tests.

At any rate, FreeBSD's local changes have already been upstreamed.