sysutils/deboostrap: make it use bash, mount fdescfs,
ignore --configure errors

For some reason, when run with stock sh(1) the script aborts just
after 'Unpacking the base system'.

The fdescfs mount fixes 'dpkg --configure' for rsyslog. Note that
there are also some kernel patches required to make it work completely.

The fdescfs is not enough to fix everything, though, so just ignore
'dpkg --configure' errors for Bionic; they don't break anything important.

PR: ports/247698
Reviewed by: 0mp (earlier version)
Approved by: maintainer timeout (2 weeks)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D25594