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