cron: consume blanks in system crontabs before options

On system crontabs, multiple blanks are not being consumed after reading the
username. This change adds blank consumption before parsing any -[qn] options.
Without this change, an entry like:

• * * * * username -n true # Two spaces between username and option.

will fail, as the shell will try to execute (' -n true'), while an entry like:

• * * * * username -n true # One space between username and option.

works as expected (executes 'true').

For user crontabs, this is not an issue as the preceding (day of week
or @shortcut) processing consumes any leading whitespace.

PR: 253699
Submitted by: Eric A. Borisch <eborisch@gmail.com>
MFC after: 1 week

(cherry picked from commit 37cd6c20dbcf251e38d6dfb9d3e02022941f6fc7)