Fix timer accuracy

[fjtrujy]

Description

This PR fixes an existing issue #530.

The existing timer implementation was working almost right, the real issue was in the implementation _times in libcglue that was losing too much precision.
Additional changes have been made too:

• Use TimerBusClock2USec for returning proper usecs in the _times function
• Improve errno logic in the nanosleep function.
• Improve the accuracy of the timer by reducing the GAP after the immediate equal interruption.
• Improve variable names and comments in the timer.
• Put back the error tolerance in the nanosleep example to 5ms as it was originally.

Additionally, I have also tested implementation with the program described here: #527 (comment)

Bigger diff: 24, requested: 9145, got: 9169

So everything looks to be right 👏

Closes #530

[fjtrujy]

Pinging @davidgfnet