Extract process-management functionality into a new lib/process

Note that procmon does *not* go here, since procmon needs to
integrate with the event loop.

src/lib/process/.may_include

@@ -0,0 +1,15 @@
+orconfig.h
+
+lib/cc/*.h
+lib/container/*.h
+lib/ctime/*.h
+lib/err/*.h
+lib/fs/*.h
+lib/log/*.h
+lib/malloc/*.h
+lib/process/*.h
+lib/string/*.h
+lib/testsupport/*.h
+lib/thread/*.h
+
+ht.h