Use strtod, not atof, for parsing doubles in the configuration.

[nmathewson]

This lets us detect erroneous doubles, which previously we could not
do.

Fixes bug 31475; bugfix on commit 00a9e37, a.k.a svn:r136.

[coveralls]

Pull Request Test Coverage Report for Build 6192

• 10 of 10 (100.0%) changed or added relevant lines in 1 file are covered.
• 528 unchanged lines in 6 files lost coverage.
• Overall coverage increased (+0.06%) to 63.229%

---

Files with Coverage Reduction	New Missed Lines	%
src/feature/client/entrynodes.c	2	86.25%
src/lib/malloc/map_anon.c	4	88.24%
src/lib/err/torerr.c	13	82.93%
src/lib/log/util_bug.c	16	50.0%
src/lib/log/log.c	127	71.61%
src/app/config/config.c	366	73.67%

Totals
Change from base Build 6120:	0.06%
Covered Lines:	47852
Relevant Lines:	75680

---

💛 - Coveralls

[dgoulet-tor]

The endptr == value check I believe needs to be checked also along with v == 0. Man page:

If no conversion is performed, zero is returned and (unless endptr is
null) the value of nptr is stored in the location referenced by endptr.

The other thing I would probably look for is the overflow situation? It is a bit unclear if this current check would catch that (endptr == value) but manpage gives us clear values to look at in that case:

   If the correct value would  cause  overflow,  plus  or  minus  HUGE_VAL
   (HUGE_VALF,  HUGE_VALL)  is  returned  (according  to  the  sign of the
   value), and ERANGE is stored in errno. If  the  correct  value  would
   cause underflow, zero is returned and ERANGE is stored in errno.