Some BOINC Manager Computing Preferences need better sanitization

[JacobKlein]

Some BOINC Manager Computing Preferences need better sanitization

• "Idle minutes" preference input should not allow negative numbers
  - "Hours" inputs should not allows entries like !13:239
  - "% of the processors" should not allow negative numbers or numbers >100
  - "% CPU Time" should not allow negative numbers or numbers >100
  - "While processor usage is less than" should not allow negative numbers or numbers >100
  - "Maximum download rate" should not allow negative numbers
  - "Maximum upload rate" should not allow negative numbers

I don't like the idea of the program "assuming" it knows what I meant, so... if I type a negative number, it should not automatically treat it as positive.  It should prevent me from saving the bad input.