Declare various sig_ macros as nogil

[jdemeyer]

One consequence is to allow Cython's OpenMP facilities, see #12690.

While we're at it, do lots of clean-up of signal/error handling after #9640 and friends:

• use sig_error() for NTL and libGAP
• remove the old _sig_on macros which are deprecated since 3 years

Follow-up at #15543: update documentation

[pbruin]

Looks good and all tests pass.

[vbraun]

Conflicts with the new developer manual, please resolve...

[jdemeyer]

What was the point of introducing changes like

-When writing Cython code for Sage, special care must be taken to ensure
-the code can be interrupted with ``CTRL-C``.
-Since Cython optimizes for speed,
-Cython normally does not check for interrupts.
+When writing Cython code for Sage, special care must be taken to
+ensure the code can be interrupted with ``CTRL-C``.  Since Cython
+optimizes for speed, Cython normally does not check for interrupts.

Seriously, that's just asking for merge conflicts...

[jdemeyer]

This is even worse:

-A common mistake is to put ``sig_off()`` towards the end of a
+A common mqistake is to put ``sig_off()`` towards the end of a

(from 67a76032fda3ee140c3bc52422020ef722cbee52)

[jdemeyer]

Wow, this bad: commit 67a76032fda3ee140c3bc52422020ef722cbee52 reverts all changes to coding_in_cython.rst done by #13311. How did that happen?

-    sage: alarm(0.5); factor(10^1000 + 3)
-    Traceback (most recent call last):
-    ...
-    AlarmInterrupt
+    sage: import sage.tests.interrupt
+    sage: try:
+    ...     sage.tests.interrupt.interrupt_after_delay()
+    ...     factor(10^1000 + 3)
+    ... except KeyboardInterrupt:
+    ...     print "ok!"
+    ok!
+

[jdemeyer]

Volker, apologies for my bad mood. I could easily revert the changes from 67a76032fda3ee140c3bc52422020ef722cbee52 which conflict with this patch, but I guess you should tell me whether that's a good idea.

[jdemeyer]

I moved the documentation part to #15543. The git branch contains all changes from the Mercurial patch except for the documentation.

[jdemeyer]

I moved the documentation part to #15543. The git branch contains all changes from the Mercurial patch except for the documentation.

---

New commits:

​463bffe

Declare various sig_ macros as nogil

[vbraun]

The changes to the developer guide were lost because I had forked that off to rewrite it, sorry. I think the changes from #13311 were the only ones affected.