coverage: Exclude generated static proposal keywords

The lines in the gperf-generated proposal_keywords_static.c are now
mapped to the (much shorter) .txt source file, which causes mismatches
like these:

  genhtml: ERROR: no data for line:190, TLA:GNC, file:/home/runner/work/strongswan/strongswan/src/libstrongswan/crypto/proposal/proposal_keywords_static.txt

We could ignore "unmapped" errors in genhtml, but since the file is
generated anyway, we can also exclude it from the results and still
get such errors in case this happens for other files.  Another alternative
would be to remove the `#line` macros in the generated file.  Then the
coverage of the actual C file would get reported (but again, it's
generated, so there isn't much value in it).

Also updated the branch coverage option as the one with `lcov_` prefix
is deprecated.

Makefile.am

@@ -65,10 +65,11 @@ cov-reset: cov-reset-common
 cov-report:
 		@mkdir $(top_builddir)/coverage
 		lcov -c -o $(top_builddir)/coverage/coverage.info -d $(top_builddir) \
-			 --rc lcov_branch_coverage=1
+			 --rc branch_coverage=1
 		lcov -r $(top_builddir)/coverage/coverage.info '*/tests/*' '*/suites/*' '/usr*' \
+			 '*proposal_keywords_static.*' \
 			 -o $(abs_top_builddir)/coverage/coverage.cleaned.info \
-			 --rc lcov_branch_coverage=1
+			 --rc branch_coverage=1
 		genhtml --num-spaces 4 --legend --branch-coverage --ignore-errors source \
 				-t "$(PACKAGE_STRING)" \
 				-o $(top_builddir)/coverage/html \