sys-process/cronbase: avoid bash complaining about null bytes

sys-process/cronbase: avoid bash complaining about null bytes

Newer versions of bash warn about null bytes in command substitutions,
so run-crons would generate mails with messages like this:

/usr/sbin/run-crons: line 59: warning: command substitution: ignored null byte in input
/usr/sbin/run-crons: line 59: warning: command substitution: ignored null byte in input
/usr/sbin/run-crons: line 60: warning: command substitution: ignored null byte in input
/usr/sbin/run-crons: line 60: warning: command substitution: ignored null byte in input

Since the warning is not useful and the resultant string has its null
bytes deleted, we might as well delete them with tr and get rid of the
warning.