Fix waring: using abs() for floating point type

src/flowgrind.c:1336:10: warning: using integer absolute value function 'abs'
when argument is of floating point type [-Wabsolute-value]
        while ((abs(value) / (dez - 1.0)) > 1.0) {
                ^
src/flowgrind.c:1336:10: note: use function 'fabs' instead
        while ((abs(value) / (dez - 1.0)) > 1.0) {
                ^~~
                fabs
Revert "flowgrindd: fix compiler warning"

This reverts commit 6f969b5245c7f93d90a8d0452a2cdd66bd53627c.
Because the change caused the following warning on FreeBSD:

src/daemon.c:1072:30: warning: format specifies type 'size_t' (aka 'unsigned long') but the argument has type 'socklen_t' (aka 'unsigned int') [-Wformat]
                flow->id, cmsg->cmsg_type, cmsg->cmsg_len);
                                           ^~~~~~~~~~~~~~
./src/debug.h:53:46: note: expanded from macro 'DEBUG_MSG'
                        (unsigned int)pthread_self()%USHRT_MAX, ##__VA_ARGS__);\
flowgrindd: properly fix compiler warning

Apparently cmsg_len in defined as size_t in some newer glibc versions,
so cast it to socklen_t in order to fix the compiler warning.
Merge pull request #169 from alexander-zimmermann/clang-warning

Fix warning: using abs() for floating point type