Restore fixes reverted by r211607:

r209631: Use MIUtilSystemLinux on FreeBSD as well

  We should later rename this file (probably MIUtilSystemPOSIX), but
  more clean-up is still needed here, and we can wait until we better
  understand how this code may be shared between FreeBSD, Linux, and OS X.

r209632: Add stdlib.h for malloc and friends

r209633: Remove include of obsolete stropts.h header

  The header is for POSIX streams functionality, and does not exist on
  FreeBSD, OS X, or contemporary Linux distributions.

Issue reported by John Wolfe.