List supported coverage criteria in CBMC's help output

Rather than making the user read further documentation, list all
(currently) supported coverage criteria when running cbmc --help.

For consistency, also add "assumes" as an alternative way to request
coverage of assume statements (previously only "assume" was supported,
even though we otherwise included the plural variants where applicable).

Author: tautschnig

src/goto-instrument/cover.cpp

@@ -130,7 +130,7 @@ parse_coverage_criterion(const std::string &criterion_string)
     c = coverage_criteriont::MCDC;
   else if(criterion_string == "cover")
     c = coverage_criteriont::COVER;
-  else if(criterion_string == "assume")
+  else if(criterion_string == "assume" || criterion_string == "assumes")
     c = coverage_criteriont::ASSUME;
   else
   {

src/goto-instrument/cover.h

@@ -31,7 +31,12 @@ class optionst;
 
 #define HELP_COVER                                                             \
   " --cover CC                   create test-suite with coverage criterion "   \
-  "CC\n"                                                                       \
+  "CC,\n"                                                                      \
+  "                              where CC is one of assertion[s], "            \
+  "assume[s],\n"                                                               \
+  "                              branch[es], condition[s], cover, "            \
+  "decision[s],\n"                                                             \
+  "                              location[s], or mcdc\n"                       \
   " --cover-failed-assertions    do not stop coverage checking at failed "     \
   "assertions\n"                                                               \
   "                              (this is the default for --cover "            \