Change remaining StaticAssertStmt() to StaticAssertDecl()

Change remaining StaticAssertStmt() to StaticAssertDecl()

This completes the work started by commit 75f49221c22.

In basebackup.c, changing the StaticAssertStmt to StaticAssertDecl
results in having the same StaticAssertDecl() in 2 functions.  So, it
makes more sense to move it to file scope instead.

Also, as it depends on some computations based on 2 tar blocks, define
TAR_NUM_TERMINATION_BLOCKS.

In deadlock.c, change the StaticAssertStmt to StaticAssertDecl and
keep it in the function scope.  Add new braces to avoid warning from
-Wdeclaration-after-statement.

In aset.c, change the StaticAssertStmt to StaticAssertDecl and move it
to file scope.

Finally, update the comments in c.h a bit.

Author: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Co-authored-by: Peter Eisentraut <peter@eisentraut.org>
Discussion: https://www.postgresql.org/message-id/aYH6ii46AvGVCB84%40ip-10-97-1-34.eu-west-3.compute.internal