CPU_ISSET may return any integer greater than 1, casting is needed.
Also, simplify the error checking with asserts.

Backtrace functions are GNU extensions.

Close #93

Timestamp last_borrow should only be updated when
the borrow does not succeed

Fix typo
Add and update some comments

Options parsing was not correctly removing "=values"

This option will make warnings to raise a DLB panic error

Function strstr cannot be used to remove tokens since it
may remove options that contain a substring of other option

Fix #89

- rebind also from the beginning
- preserve previous binding if possible
- do no set dirty flag if no thread needs rebinding

Optimize shmem functions that iterate all CPUs looking for
available CPUs by comparing the timestamps of the last borrow
and the last lent.