fix determining shell in bootstrap_startscript

[boegel]

Take into account that $SHELL may contain something like '/bin/bash --noprofile --norc'

This is probably worth pushing back upstream...

[boegel]

Without this fix:

$ export SHELL=/bin/bash
$ echo "${SHELL##*/}"
bash

$ export SHELL="/bin/bash --no-profile --norc"
$ echo "${SHELL##*/}"
bash --no-profile --norc
# this causes trouble, results in running "emerge bash --noprofile --norc", which fails

With this fix:

$ export SHELL="/bin/bash --no-profile --norc"
$ echo $(echo "${SHELL##*/}" | cut -f1 -d' ')
bash
$ export SHELL=/bin/bash
$ echo $(echo "${SHELL##*/}" | cut -f1 -d' ')
bash