Rather than using fork+pipe+system+waitpid, most of which are only
available on POSIX-like systems, use popen which is also available on
Windows (under the name _popen).

Changes since v1:
- Create fread_noeintr to avoid EINTR in fread without reading any data.
- Handle short reads from fread.  This can happen with non-conformant
  libc or if EINTR occurs after reading some data.
- Define _POSIX_C_SOURCE for popen/pclose with strict implementations
  which require it (e.g. gcc with -std=c11).

Changes since v2:
- Revert fread_noeintr and short read changes in v1 as unnecessary.
Signed-off-by: Kevin Locke <kevin@kevinlocke.name>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>