Replace makefs' hand-rolled unescaping with strunvis

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.

Reviewed by: marcel
MFC after: 2 weeks
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D12104