Replace makefs' hand-rolled unescaping with strunvis

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