add: create Singular/rlimit.{c,h} for setting resource limits
(cherry picked from commit 88e216d8bea3cefdff011bacb24458c88bd1eb32)

Signed-off-by: Andreas Steenpass <steenpass@mathematik.uni-kl.de>
chg: raise RLIMIT_NPROC if necessary for fork()
(cherry picked from commit 4f397cf7bc1b07f8c0dd8cab10d1311de491db21)

Signed-off-by: Andreas Steenpass <steenpass@mathematik.uni-kl.de>
Merge pull request #419 from steenpass/rlimit_sw

raise RLIMIT_NPROC if necessary for fork()