Various fixes to the "config and tuning" section of the Handbook.

• Reword the paragraph before a set of tunables for Linux, as it wasa bit confusing.
• Correct the name of 'kern.maxfiles' (it was 'kern.maxfile') before, and reword the text around it to remove the need for a possessive "'s", which would look funny after 'maxfiles'.
• Finish properly a sentence which was missing a fullstop.

Submitted by: ganbold &at; micom ˙ mng ˙ net