smartd.cpp, popen_as_ugid.cpp: Don't use 'getdtablesize()'.

This function is declared 'legacy' since SUS 1997 and no longer part
of POSIX since 2004.  Use 'sysconf(_SC_OPEN_MAX)' instead.
This fixes build on android (GH issues/142).

git-svn-id: https://svn.code.sf.net/p/smartmontools/code/trunk@5402 4ea69e1a-61f1-4043-bf83-b5c94c648137