timeout: remove dependence on libm

timeout: remove dependence on libm

This was seen to add about 100,000 ns to the startup time,
on a 2.6 GHz i7-5600U with glibc 2.40.

* .gitignore: Remove /lib/fenv.h.
* bootstrap.conf: Remove fenv-rounding and signbit deps.
* src/local.mk: Remove fenv lib dependency.
* src/timeout.c (is_negative): A new helper function to
be equivalent of signbit in the underflow case.
(parse_duration): Remove the rounding up logic,
as a nanosecond here or there has no significance.