Update to r45559:

Replace all occurances of 'pkgng' with 'pkg' in the handbook
Kept existing xml:id's to avoid breaking external links to the
documentation
Sanitized the documentation of references to pkg_add
Fix an incorrect variable name: daily_backup_pkgng_enable should be
daily_backup_pkgdb_enable
Fix a linebreak in the middle of a variable name
Clarify that pkg2ng is not a required step on newer systems