Use strlen(d.d_name) instead of d_namlen

[Callum Davies]

In src/kmk/kmkbuiltin/fts.c, use strlen(d.d_name) instead of d_namlen

d_namlen is a non-standard member of struct dirent, used primarily on *BSD. It has the same value as strlen(dirent.d_name), which is available everywhere, so we might as well use that.

One of the ifdefs removed in this patch prevents fts.c from compiling on Linux systems that aren't using glibc.