Prevent Squid forcing -b 2048 into the arguments for sslcrtd_program

Prevent Squid forcing -b 2048 into the arguments for sslcrtd_program

Previously Squid assumed it was running with the default sslcrtd_program, which
takes an argument for the FS block size. This causes issues for administrators
that use their own helpers that happen to take a -b argument that means
something else entirely, causing confusion and preventing them from removing
this argument.

A summary of the changes:

* Move the block size retrieval from Squid into security_file_certgen. It
  does not use fsBlockSize as that introduces a lot of dependencies on
  unrelated Squid code, e.g. fde, Debug, MemBuf.

* Make the -b argument mostly redundant, but leave it there so
  administrators can overrule xstatvfs.

* Fix a small typo.

This work is submitted on behalf of Bloomberg L.P.