Removed duplicate headers imports

Review: https://reviews.apache.org/r/25042
Replaced PKG_CHECK_MODULES with AC_CHECKs in configure.ac.

Review: https://reviews.apache.org/r/25030
Replaced PKG_CHECK_MODULES with AC_CHECKs in libprocess configure.ac.

Review: https://reviews.apache.org/r/25031
Merge branch 'master' into disk_isolation_containerization