when --multi-prog is in use.

The slurmstepd now parses the multi-prog file earlier and keeps track
of the individual argv array for each task.  Searching the PATH
environment variable is still done just before task exec() time so that
the search is done as the user and not as root.

Major enhancements to all multi-core code, bug fixes, performance improvements, etc.

VER### in header.

for multi-prog runs.

Basically works, but needs cleanup and optimization (especially on the
server side).

Also, [srun|slaunch]/multi_prog.c are clearly buggy.

ported as necessary to v1.2.

consistency between the commands.

Convert socket/core/thread info from uint32_t to uint16_t to save some space.