Add trailing letter b to bsdtar substitute pattern (#2012)

Add trailing letter b to bsdtar substitute pattern (#2012)

The letter b stands for "from (b)eginning" and specifies that a
substitute expression should be matched from the beginning of the
string, regardless if and where a previous substitute expression
matched.

Example:
Transform filename from B-A to A-B and remove all underscores.

Attempt without option b:

    bsdtar -cft -s '/\(.*\)-\(.*\)/\2-\1/gp' -s "/_//g" ab_c-d_ef
    ab_c-d_ef >> d_ef-ab_c

With option b:

    bsdtar -cft -s '/\(.*\)-\(.*\)/\2-\1/gp' -s "/_//gb" ab_c-d_ef
    ab_c-d_ef >> def-abc