cpplint should warn about any use of exit() or abort()

[tautschnig]

Following @martin-cs' suggestion in #1875:

Suggested policy: The only way for normal termination of program execution is via return from main. Abnormal termination is to be handled via the macros provided by invariant.h. Any other use requires explanation and should be explicitly marked as NOLINT.

[martin-cs]

Slight tweak.

Suggested policy: The only way for normal termination of program execution is via return from main. Explicit abnormal termination should be done via throw, invariant failure (via the macros provided by invariant.h) should also result in abnormal termination. Any other use requires explanation and should be explicitly marked as NOLINT.

[peterschrammel]

See also #1314