WrapperExecLock fix.
Allow plugins to call fork/exec during preCkpt etc. events.
Skip protected-fds during fd-restore.

Otherwise we might accidentally step over the coordinator fd etc.
File save heuristics updated.
Added .gitignore file.
Minor updates to .gitignore.
- Remove compiler warning: moved definition of MTCP_SYS_GET_SET_THREAD_AREA
    from mtcp_sys.h to mtcp.c
- Minor polishing
Bunch of fixes detected by coverity.
MTCP: Properly dup() stdin/out fds for gzip child.
Added wrappers for dup/dup2/dup3.
Root process pids added to SharedData
Construct UniquePid from ckpt filename.
Add wrappers for openat/opendir/closedir.

They will be needed by the Connection plugin.
Implement Connection plugin as a true plugin

- The checkpoint image is just a memory dump of the process. All the
information required to recreate child processes and file descriptors is
now part of the process memory. This information used to be part of
"DMTCP-header" in the checkpoint image. Now this is part of the memory
dump.

- During restart, the child process are forked off only after the
process's memory has been restored. Once the child processes have been
forked, each process restores/recreates the file descriptors including
socket connections for which it was ckpt-leader. The file descriptors
are then passed to non-leader processes over UNIX domain sockets.

- Socketpair aren't properly restored at this time. Instead, they are
restored as AF_INET sockets.

- make check-screen is current failing due to a bug related to
packet-mode detection for pseudo-terminals.
--enable-fast-restart for mmap based restart.

Restarting a checkpoint image, created without any sort of compression,
can be done via mmap making restarts faster for large images.

This removes the previous --enable-fast-ckpt-restart option.
Torque plugin created.
Sockets use NameService for peer discovery.
Disentangle Connection plugin from dmtcphijack.

- ConnectionIdentifier moved to connection plugin.
- Removed --external-socket-handling and --enable-condor-support
  configure flags.
Some bug-fixes for SharedData.
Bugfix for Util::{lock,unlock}File().