chg: port commit 7e0ad82 from master to spielwiese

commit hash in master: 7e0ad82686aab981653d1f5661ed4e7c730eaea9
was: 'chg: remove SSI_BLOCK_CHLD and SSI_UNBLOCK_CHLD'

The changes in the other two files affected by this commit have already been
ported to spielwiese.
chg: port commit e9da36a from master to spielwiese

commit hash in master: e9da36afd8cdf7d84cd5f977ba1d25e86584165b
was: 'chg: in slCleanUp(), kill the link before freeing the memory'
chg: port commit 129ed66 from master to spielwiese

commit hash in master: 129ed66a74c2b5b0201c02efbc76fa49062d2fd2
was: 'rmv: remove attribute __attribute__((noreturn)) from m2_end()'
chg: port commit f860657 from master to spielwiese

commit hash in master: f860657053f1c0d3cdf380bc47fbd8fe81f49577
was: 'chg: remove special SIGCHLD handling for HPUX_9'

The file Singular/mpsr_sl.cc, which was also affected by this commit, has
been deleted anyway, see d828d63.
rmv: remove parallel_l.tst and realclassify_l.tst

These files are almost identical to parallel_s.tst and realclassify_s.tst,
but they are not used for the tests. Already removed in master.
Merge pull request #365 from steenpass/changes_from_master

Changes from master (links, signals)